Louis Imbert

ENDORSING MIGRATION POLICIES IN CONSTITUTIONAL TERMS: THE CASE OF THE FRENCH CONSTITUTIONAL COUNCIL

Keywords: immigration, French Constitutional Council, constitutional review, methods of reasoning, constitutional reasoning, constitutional rights

This article sets out to inquire into the French Constitutional Council's approach when dealing with immigration matters. It seeks to demonstrate how the Council's case law has endorsed, for the most part, the legislator's immigration policies, recognizing extensive police powers and striking down only the most excessive provisions of immigration laws. It is argued here that the Council's seemingly neutral methods of reasoning are in fact politically oriented instruments providing stable support for restrictive immigration policy preferences. An overall analysis of the Council's case law sheds critical light on the main methods of reasoning advanced by the Council to endorse immigration policies, even in their most recent restrictive trends. The Council has clearly opted in favor of stricter immigration control, deliberately rejecting a rights-based approach.

I. INTRODUCTION

As in neighboring countries, immigration policies in France have taken a repressive turn in the past few decades, focusing more and more on the 'fight against irregular migration', both at borders and inside the country. The constant pace of legislative reforms is revealing in this regard. Almost every Government since 1980 has modified aspects of immigration law, mostly following a restrictive trend. In this context, the role of French courts appears more essential than ever. In the early 1970s, the Conseil constitutionnel (French Constitutional Council) began taking on responsibility for the protection of fundamental rights guaranteed by the French Constitution (and related documents). However, although the Council confirmed early on that étrangers (foreigners) are protected by the Constitution, 1 it has also been inclined to maintain an important margin of action for the legislator and the administration, on the basis of increasingly significant public order considerations.

This article sets out to inquire into the French Constitutional Council's approach when dealing with immigration matters. It seeks to demonstrate how the Council's case law has endorsed, for the most part, the legislator's immigration policies, recognizing extensive police powers and striking down only the most excessive provisions of immigration laws. The Council's role in confirming the main paradigms of French immigration policies can be understood first and foremost as one of translating the legislator's policies into constitutional terms.

While there are critical commentaries of specific cases pointing this out, 2 as well as a few systematic analyses of the earlier case law of the Council, 3 there is a strong need for an updated critical analysis of the Council's case law on immigration policies. This article aims at filling this significant academic gap by providing a critical overall analysis of the Council's case law on immigration matters (53 decisions since 1980). The analysis sheds critical light on the main methods of reasoning followed by the Council to endorse immigration policies, even in their most recent restrictive trends. It is argued here, from a legal realist perspective, that the seemingly neutral methods of reasoning used by the Council are in fact politically oriented instruments providing stable support for restrictive immigration policy preferences. This article is divided into four parts. Part II will introduce the historical, political, institutional and legal context in which the Council has reviewed immigration policies. Part III will deal with the ways in which the Council has generally endorsed immigration policies in constitutional terms. Part IV will demonstrate that the Council has loosened its protection standards throughout the past decades, especially when dealing with immigration detention regimes, which have been significantly expanded since their creation in the early 1980s. Part V will provide some concluding remarks.

2 See e.g. Serge Slama, 'Les lambeaux de la protection constitutionnelle des étrangers' (2012) 90 Revue française de droit constitutionnel 373-386. 3 Bruno Genevois, 'Le Conseil constitutionnel et les étrangers' in Xavier Robert (ed), Mélanges Jacques Robert (Montchrestien 1998) 253-277; Olivier Lecucq, 'Le statut constitutionnel des étrangers en situation irrégulière' (LLD thesis, Université d'Aix-marseille 1999); Raymond Coulon, Des droits de l'homme en peau de chagrin. Le droit des étrangers dans la jurisprudence du Conseil constitutionnel (L'Harmattan 2000); Justin Kissangoula, La Constitution française et les étrangers. Recherches sur les titulaires des droits et libertés de la Constitution sociale (Librairie générale de droit et de jurisprudence 2001).

II. CONTEXT OF THE CONSTITUTIONAL COUNCIL'S CASE LAW

This second part will introduce some basic context on the recent history and politics of immigration, the rise of the Constitutional Council as the potential guardian of fundamental rights in France and the indeterminacy of the French Constitution as regards the status of foreigners. This context will help illustrate the role of the Constitutional Council when reviewing immigration laws. It will demonstrate that the Council enjoys a rather wide margin of action on immigration matters as it is not particularly constrained by the Constitution in this regard.

Historical and Political Context: Contested Immigration Policies

Like other Western countries, France has a long history of policies aimed at controlling human mobility. 4 Until the 19 th century, such policies mostly focused on vagrant and indigent individuals, whether French or foreign. 5 Borders were local and national, as passports were required both internally and externally. 6 It was only in the second half of the 19 th century that the "immigration problem" emerged and that policies aimed at foreigners as such -understood unambiguously as non-nationals rather than as mere outsiders -started to proliferate. 7 An 1849 law consolidated the French administration's power of expulsion, the deportation of foreigners for reasons In this regard, there are striking parallels with policies in place at the time in other countries. For a comparative perspective, see Fahrmeir, Faron and Weil (n 4). On the emblematic case of the United States, see in particular Kunal M. Parker, Making Foreigners: Immigration and Citizenship Law in America, 1600-2000 (Cambridge University Press 2015). of public order. 8 Between the late 1880s and the 1920s, other specific pieces of legislation were adopted to increase control over foreigners. Following the adoption of a decree in 1888, foreigners had to declare their residence to local authorities. 9 From 1917 onwards, they were required to obtain an identity card from the préfet (prefect) and to declare their first residence and any subsequent movement within the country, including any change of residence. 10 The 1930s witnessed a new wave of restrictive laws, in the context of a devastating economic crisis and rising xenophobia and antisemitism. A décret-loi (decree-law) adopted in May 1938 enacted more repressive immigration control measures, in addition to assembling existing ones in an unprecedented effort to offer a general legislative framework. 11 After the Second World War, a new general framework was adopted. 12 It would become the basis for contemporary French immigration law, which was eventually codified in the mid-2000s as CESEDA. From 1945 to the late 1960s, France was in dire need of foreign labor. Authorities seldom enforced immigration control measures and foreigners were often able to obtain documentation once they had arrived in France. 13 In the early 1970s, as the country entered a lengthy economic crisis (related to the oil shock) and suffered massive unemployment, authorities announced their intention to close borders. Regulations were adopted to block labor and family migration and later on to foster or even force the departure of foreigners residing in 8 See articles 7 and 8 of the loi du 3 décembre 1849 sur la naturalisation et le séjour des étrangers en France. For a brief summary of previous legislation, see Danièle Lochak and François Julien-Laferrière, 'Les expulsions entre la politique et le droit ' (1990) 12 Archives de politique criminelle 75. France.14 A decree was successfully challenged before the Conseil d'État (Council of State), which recognized in a 1978 landmark decision that foreigners were entitled to the right to lead a normal family life. 15 This was an important step given the Council of State's longstanding reluctance to review the substance of immigration control measures. 16 With immigration becoming an increasingly heated political issue, legal reforms have been adopted almost every two to three years since 1980. 17 Such reforms have mostly followed a restrictive trend, imposing more and more conditions on the residence of foreigners in France, reducing legal entry pathways as well as procedural and substantial guarantees and facilitating and aggravating deportation and detention measures. 18 In general, French immigration policies, as pursued by the legislator and the administration, have tended to focus more and more on the 'lutte contre l'immigration irrégulière' ('fight against irregular migration'), while preserving the right to asylum only to a minimal extent. 19 Recently, greater emphasis has been placed on 'integration', a vague term crystallizing ideological debates regarding the place of immigrants in France. 20 These debates rely, at least in part, on the widely held misconception that the proportion of foreigners has increased significantly over the course of the last century in France. Although the current proportion (7.6 per cent in 2020) is the highest attained over the past century, it is only slightly above 1931 and 1982 levels (6.6 and 6.8 per cent, respectively). 21 In the past few decades, MPs and Senators have lodged numerous challenges before the Constitutional Council, mostly arguing that certain laws violated the constitutional rights of foreigners. 22 policies on constitutional grounds. 24 Hence the necessity to examine carefully the approach followed by the Constitutional Council in order to determine the role it has played in the governance of migration policies.

Institutional Context: The Rise of the Constitutional Council

A general introduction to the Constitutional Council and its rise is necessary to understand the role it plays in French jurisprudence. Created by the 1958 Constitution which founded the current Fifth Republic, the Council is composed of two types of members. 25 Nine ordinary members are appointed for nine-year non-renewable terms by three different political figures (three by the President of the Republic, three by the President of the Senate and three by the President of the National Assembly). A third of them are renewed every three years. A significant number of these ordinary members are former politicians who have served as members of Parliament and/or as government ministers. 26 In addition to the ordinary members, former Presidents of the Republic automatically become membres de droit (ex officio members) of the Council. However, not all of them have sat on the Council. 27 The Constitutional Council was created primarily to protect the prerogatives of the executive power, which had been reinforced by the Constitution of the Fifth Republic. 28 Hence the institution's designation as a Council rather than a Court, which also reflects the traditional hostility towards judges in French legal culture. Initially, the Council could only review legislative bills before their promulgation, upon referrals by the President of the Republic, the Prime Minister, the President of the Senate or the President of the National Assembly.29 However, the Council later acquired new powers, both through constitutional reforms and on its own initiative.

24

The European Court of Human Rights and the European Court of Justice have also emerged as significant battlegrounds for challenges to French immigration laws. In a landmark 1971 decision, the Council itself expanded its normes de référence (reference standards) beyond the four corners of the 1958 Constitution, resolving to consider also the documents referenced in the first paragraph of the Preamble to the 1958 Constitution. 30 Following this decision, the bloc de constitutionnalité (an expression used by constitutional law experts such as Louis Favoreu to designate the set of norms holding constitutional value in France) gradually expanded to include the 1789 Declaration of the Rights of Man and of the Citizen, the Preamble to the 1946 Constitution and the Charter for the Environment, as well as various principles and objectives of constitutional value. 31 This led to a much more substantial kind of review, extending inter alia to questions of fundamental rights protection.

Meanwhile, in 1974, a constitutional reform authorized referrals by any group of sixty MPs or Senators. 32 This became a new tool for opposition MPs and Senators, sparking an increase in referrals. This trend was reinforced by a major constitutional reform adopted in 2008 which opened a new preliminary ruling mechanism, the question prioritaire de constitutionnalité (QPC), to all litigants. 33 This procedure now allows any litigant, under certain conditions set by a 2009 organic law, to challenge the conformity of a legal provision to the rights and freedoms guaranteed by the Constitution. Admissible requests are transferred by lower courts to the competent supreme court (Council of State or Court of Cassation), which may refer the matter to the Constitutional Council. This mechanism marked a significant change for at least three reasons. First, it opened up the possibility for litigants (including foreigners) 35 to challenge the constitutionality of laws, albeit only on the basis of rights guaranteed by the Constitution. 36 this new mechanism allowed litigants to challenge the constitutionality of laws after their promulgation. Previously, once a law was promulgated, it was nearly impossible to challenge on constitutional grounds. Under this new procedure, a wide range of laws previously immune from challenge could now be brought under scrutiny by litigants. Third, as had been expected, this constitutional reform led to a further increase in the number of decisions rendered by the Council. 37 All these changes have reinforced the potential role of the Constitutional Council as a guardian of the rights of foreigners.

Legal Context: Constitutional Indeterminacy Regarding the Status of Foreigners

There are only two references to foreigners in the 1958 Constitution, despite the country's immigration history and the prior existence of a general legislative framework on the admission and residence of foreigners. The only explicit reference can be found in article 53-1 of the 1958 Constitution. 38 The other reference is implicit, incorporated by reference to paragraph 4 of the Preamble to the 1946 Constitution. 39 Both of these provisions pertain to the right to asylum, as they regulate the status of 'any man persecuted in virtue of his actions in favor of freedom '. 40 37 Hamon and Troper (n 26) 842.

38

'The Republic may enter into agreements with European States which are bound by undertakings identical with its own in matters of asylum and the protection of human rights and fundamental freedoms, for the purpose of determining their respective jurisdiction as regards requests for asylum submitted to them. However, even if the request does not fall within their jurisdiction under the terms of such agreements, the authorities of the Republic shall remain empowered to grant asylum to any foreigner who is persecuted for his action in pursuit of freedom or who seeks the protection of France on other grounds'. Unless otherwise stated, translations of constitutional provisions are those provided on the English version of the Constitutional Council's website. The term étranger (which can also mean 'foreign' or, as part of the expression à l'étranger, 'abroad') can be found in three other provisions of the Constitution, which do not relate to the status of foreigners. Article 14 provides that 'the President of the Republic shall accredit ambassadors and envoys extraordinary to foreign powers; foreign ambassadors and envoys extraordinary shall be accredited to him'. Article 35 provides that 'the Government shall inform Parliament of its

The Constitution is otherwise silent on the status of foreigners. While the bloc de constitutionnalité contains various mentions of 'citizens', the interpretation of this term cannot be established with certainty. 41 In the rich and varied constitutional tradition of France, 42 citizens are not necessarily nationals, meaning that foreigners are not necessarily excluded from the category of citizens. However, references to nationaux français (French nationals) and to le peuple français (the French people) seem to exclude foreigners unambiguously. Foreigners do not participate in the exercise of national sovereignty, since it 'shall vest in the people, who shall exercise it through its representatives and by means of referendum'. 43 They are therefore not entitled to vote, 44 with the minor exception of citizens of the European Union, who are eligible to vote for and, under certain restrictions, hold office as members of city councils. 45 Elsewhere in the bloc de constitutionnalité, in particular in the 1789 Declaration on the Rights of Man and of the Citizen, references are made to les hommes (men), tout homme (every man), l'individu (the individual), nul (none) and decision to have the armed forces intervene abroad, at the latest three days after the beginning of said intervention'. Lastly, article 73 provides that overseas departments and regions are not authorized to determine rules regarding certain areas, including 'foreign policy'. Emphases added. chacun (each person). These expressions appear to encompass both nationals and foreigners, 46 although doubts have been raised when interpreting some of these terms. 47 Beyond this terminological matter, one searches in vain for a constitutional provision explicitly granting immigration powers to either the legislator or the executive. Although article 34 of the Constitution grants the legislator the power to set rules regarding nationality as well as 'civic rights and the fundamental guarantees granted to citizens for the exercise of their civil liberties', it confers no explicit power upon Parliament regarding immigration matters. 48 Former Constitutional Council Secretary General and Councillor of State Bruno Genevois confirms this constitutional indeterminacy as he notes that the main obstacle faced by the Constitutional Council […] resides in the fact that the constituent did not take into account the situation of foreigners. Unlike the Fundamental Charter of other European countries, there are no general provisions on non-nationals in the 1958 Constitution […]. 49 Genevois instead observes imprecision and 'great heterogeneity' within the bloc de constitutionnalité with respect to foreigners. 50 He believes that 'attempting to ground the constitutional rights of foreigners following a literal approach would have led to a great many approximations and even 46 Henri Labayle, 'Le statut constitutionnel des étrangers -Rapport français' in Pierre Bon (ed), Etudes de droit constitutionnel franco-espagnol (Economica 1994) 31.

47 Members of the Constitutional Council have expressed hesitation as to the meaning of the word 'individual', notwithstanding the fact that it seems to refer universally to all persons. During a deliberation session, Councillor Jacques Robert stated: 'Indeed, the 1946 Preamble asserts that ''the Nation provides the individual and the family with the conditions necessary to their development''. Does the nation provide these conditions to all or to nationals only? I am not sure whether everyone, whoever they are, is targeted by this provision.' Minutes of the deliberation session of 22 January 1990, 33 (my translation).

48

Articles 34 and 37 of the Constitution define the substantive scope of statutes and regulations, respectively. According to article 37, any matter falling outside of the areas listed by article 34 is subject to regulation. Following a strict interpretation of both provisions, one might therefore conclude that immigration is a matter of regulation, not statute. However, the Constitutional Council has long recognized a broad scope for statutes, leaving considerable space for legislative intervention. 49 Genevois (n 3) 254-255 (my translation).

inconsistencies.' 51 In a similar vein, Professor Henri Labayle wrote in the 1990s that the constitutional status of foreigners in France is marked by uncertainty and ambiguity. Uncertainty, […] due to the silence of the Constitution, and ambiguity, above all, regarding the place to which domestic law assigns nonnationals. While it is true that the fundamental text and the rights it guarantees should bear an identity and a project, one is forced to recognize that the French Constitution only expresses indifference, if not ignorance, when it comes to immigration law. 52 Due to this constitutional indeterminacy, the Constitutional Council has been compelled to 'fill in part of the gaps' 53 regarding the respective scope of the constitutional rights of foreigners and the immigration powers held by the legislator and the executive. This has undoubtedly provided the Council with a wide margin of action. Given these weak constitutional constraints, the Council could have followed a rights-based approach. As Parts III and IV will now show, it has instead aligned much of its case law with the immigration control priorities set by the legislator and the administration.

III. ENDORSING IMMIGRATION POLICIES ON PRINCIPLE

This third part will explore the ways in which the Constitutional Council has endorsed immigration policies as a matter of principle. The Council has done so by recognizing extensive police powers in the legislator and exercising its usual self-restraint on matters of constitutional challenge. By deeming constitutional most of the legal provisions brought under its review, the Council has expressed stable support for immigration policy preferences set by the legislator and the Government. This part will inquire into the specific methods of reasoning relied upon by the Council in support to such policy preferences.

Recognizing Extensive Police Powers

As soon as the Constitutional Council started to review immigrations laws, it recognized broad legislative powers in the field of immigration. It has provided two complementary for doing so. Both were spelled out in a landmark decision rendered on 13 August 1993, which is considered to have established the 'constitutional status of foreigners'. 54 First, foreigners can be treated differently by the legislator since they are 'placed in a different situation than that of nationals' in the immigration context. 55 Consequently, immigration rules do not violate the principle of equality, as they pursue general interest goals. This justification has been applied to uphold concrete measures criticized by members of Parliament as discriminatory. [START_REF]Cons const[END_REF] The Council has also stated that immigration measures can only affect foreigners and are therefore not discriminatory. 57 Another way in which the Council has justified the differential treatment between nationals and foreigners has been to assert that 'no principle or rule of constitutional value guarantees to foreigners general and absolute rights of access to and residence in the national territory'. 58 This formula has been reiterated by the Council many times. 59 The second way in which the Council has justified broad legislative powers in the field of immigration has consisted in stating that foreigners' 'entry and residence conditions may be restricted by administrative police measures conferring extensive powers on public authorities and relying on specific rules'. 60 This position has also been recalled in a significant number of subsequent decisions. 61 The Council has also confirmed that it is for the legislator 'to determine, in conformity with constitutional principles, and considering its public interest goals, measures applicable to foreigners' entry and residence in France'. 62 Deeming most immigration control measures part of administrative police prerogatives has had far reaching consequences which will now be explored.

A. Rejecting Criminal Law Guarantees

The first consequence of characterizing most immigration control measures as administrative police measures is that it places them outside the scope of criminal law guarantees. Akin to the United States Supreme Court's old case law considering that deportation is not 'punishment for a crime', 63 has allowed the Council to refrain from examining claims asserting a range of rights attached to criminal procedures.

The Council has determined that in the exercise of its immigration powers the legislator may resort to either criminal or non-criminal measures. 64 As regards criminal measures, the legislator is quite free to determine the definition of criminal offenses and the penalties associated with them. 65 Such measures must however respect criminal law guarantees provided for in the bloc de constitutionnalité, in particular those contained in the 1789 Declaration of the Rights of Man and of the Citizen. The legislator may also resort to noncriminal -i.e. administrative -measures. The Council rarely challenges the legislator's characterization of a measure as non-criminal. 66 As early as 1980, the Council established that expulsions (a specific type of deportation that applies to individuals considered as a serious threat to public order pursuant to article L. 631-1 of CESEDA) were 'police measures which do not follow the same objectives as criminal repression'. 67 Based on this characterization, the Council upheld a provision that 'grants the administration with the power to take an expulsion measure based on facts which may justify a criminal conviction, but for which no permanent conviction has been pronounced by the judiciary authority'. 68 The Council therefore set aside the claim made by members of Parliament that the provision would violate the presumption of For an exception, see Cons const, décision n° 93-325 DC (n 54) cons 49. The measure at issue was an interdiction du territoire (re-entry ban) that was automatically applied to individuals targeted by an arrêté de reconduite à la frontière (deportation measure). However, in 2011, the Council reversed its position and considered that an interdiction de retour (re-entry ban) was a mere police measure. Cons const, decision n° 2011-631 DC (n 59) cons 52. A few other examples will further illustrate the serious consequences of characterizing immigration measures as non-criminal. In the landmark August 1993 decision referenced above, the Council characterized refusal of entry decisions as relying on administrative police rules specific to foreigners. Accordingly, such decisions were not subject to criminal law guarantees and could be executed automatically. 69 In a March 2003 decision, the Council deemed that the withdrawal of a temporary residence card for reasons of public order was 'not a sanction but a police measure', to which the presumption of innocence therefore did not apply. 70 In a September 2018 decision, the Council refused to classify immigration detention as a criminal sanction even though the maximum length of such detention had been extended three months. 71 Consequently, it was not subject to the principle of legality. Finally, in December 2019, the Council decided that refusal of entry and border detention were not 'sanctions having the character of punishment, but police administrative measures', meaning that police interviews conducted in such connection could continue to take place without the assistance of a lawyer. 72

B. Favoring Public Order over Rights

The second and main consequence of characterizing immigration control measures as administrative police measures is that it frames the Council's inquiry as one of balancing the rights of foreigners against the safeguarding of public order, which was first recognized as an objectif à valeur constitutionnelle (objective of constitutional value) outside of the immigration context in 1982. 73 Under traditional principles of French administrative law, administrative police measures may restrict fundamental rights in order to preserve public order. The Constitutional Council took direct inspiration from the Council of State's landmark GISTI ruling of 1978 74 when, in 1986, it first decided that public order considerations could prevail over foreigners' rights. 75 Later on, the Council presented its reasoning as a process of 'conciliation' -a balancing operation pitting the public order principle against foreigners' rights. This would seem to imply that that there is no hierarchy between those two elements and that they hold the same value. However, in most cases, the Council has favored public order over foreigners' rights. Between 1986 and 2019, in the thirty-three instances in which the Council balanced public order against rights, whether explicitly or implicitly, 76 twenty-eight provisions were upheld 77 and a further three were upheld subject to réserves d'interprétation (explained below in Part III.2), 78 two were struck down. 79 Over the course of these decisions, the Council has favored public order over the right to a normal family life, the best interests of the child, the right to private life, freedom of movement (within France), the right to individual liberty (including freedom from arbitrary detention), the right to an effective remedy and the right to asylum. Though expressed through the technical language of constitutional reasoning, this configuration is not the result of a purely mechanical operation dictated by strict rationality. Rather, it appears to express a relatively stable axiological hierarchy -i.e. a hierarchy of values -according to which public order almost always prevails over foreigners' rights. 80 The concept of public order has remained quite vague throughout the Council's case law. Despite its weak textual basis, 81 it is never clearly defined and appears to cover a wide range of considerations. At times, one wonders how a specific measure under review effectively contributes to the preservation of public order. For instance, in a 2006 decision, the Council ruled, without any explanation, that the decision to extend the minimum length of time a foreigner needed to reside in France before she could bring her family over gave rise to public order considerations that trumped her right to a normal family life. 82 Furthermore, the Council has widened the concept of public order in some respects over the past two decades. Since 2003, the concept includes the 'fight against irregular migration'. 83 This expansion seems to have been a reaction to the increasing importance of this goal in the legislator's and the Government's respective immigration policies. It also seems to have been a direct response to arguments raised by the Government in its observations before the Council, which already in 1997 presented the 'fight against irregular migration' as a significant public order issue. 84 There is, however, a paradox in the postures adopted by the legislator and the Government. While they claim to 'fight against irregular migration', their reforms have actually tended to 'produce' more and more foreigners in an irregular situation. 85 This contradiction was indeed pointed out by members of Parliament in their brief to the Council regarding the same 1997 case. 86 Nevertheless, it has not prevented the 'fight against irregular migration' from becoming a recurrent argument of the Government in its observations before the Council in defense of provisions of legislative reforms. In turn, the Council appears increasingly willing to rely on this policy objective, which it has thereby translated into a fully-fledged constitutional concept.

Beyond the vague nature and broad scope of the concept of public order, concrete justifications as to why it should prevail over rights are often very limited. As is the case for much of the Constitution Council's reasoning, which is typically formalistic and somewhat opaque, 87 explanations provided are cursory and at times incomplete from a logical standpoint. After asserting that it will resort to balancing to decide on the validity of a provision, the Council sometimes immediately jumps to its conclusion or simply reformulates the content of the provision. 88 It is also not always clear whether the balancing exercise is supposed to be performed by the legislator or by the Council. The degree of review thus varies from one decision to another. proportionality tests, it typically fails to follow rigorously the three-step process to determine whether a measure is adequate, necessary and proportionate. See made the very same statement regarding its limited competence in several decisions reviewing immigration laws. 94 With the notable exception of a 1989 decision in which the Council confirmed the constitutionality of provisions that guaranteed foreigners' rights, 95 this position has mostly led the Council to refrain from striking down provisions which could be considered detrimental to foreigners' rights.

The Council has also manifested self-restraint in granting the legislator free rein to diminish fundamental guarantees it had previously adopted to ensure respect for constitutional rights. For a time, this seemed to contradict the Council's case law in other areas. Over the course of the last two decades of the 20th century, the Council issued several decisions that appeared to impose an 'effet cliquet' ('ratchet theory') whereby Parliament could only strengthen the legal guarantees of fundamental constitutional. 96 This case law had serious limitations, though, as it applied only to 'fundamental rights' (as determined by the Council itself) and it still allowed the legislator to balance these rights against principles of constitutional value. The theory was applied sporadically and eventually abandoned. 97 In any case, when it comes to reviewing immigration laws, the Council has always appeared willing to accept the placement of restrictions upon even the most fundamental constitutional rights. 98 . As early as 1986, the Council was keen to observe that its review did not consist in comparing provisions of consecutive laws, but solely in comparing the law in question with constitutional requirements. 99 This position has been reiterated in various decisions reviewing immigration laws. 100 Since 1975, the Council has also refused to examine the conformity of laws with international conventions, following a strict interpretation of articles 55 and 61 of the Constitution. 101 This position has likewise been reiterated in several decisions reviewing immigration laws, 102 and indeed has significant consequences in this area of law, where international treaties can at times provide more protective guarantees than the domestic normative framework. For instance, the Council has refused to review the conformity of immigration laws with the 1951 Geneva Convention Relating to the Status of Refugees 103 and the 1950 European Convention on Human Rights. 104 While the Council has indirectly recognized some international norms through its interpretation of related constitutional norms (e.g. right to asylum, best interests of the child, respect for family life), it has nonetheless 98 One minor exception is the right to asylum, which was considered a fundamental right in a 1993 decision under the short-lived ratchet theory (Cons const, décision n° 93-325 DC (n 54) cons 81). However, this was not reiterated in later decisions and the Council has since confirmed the constitutionality of various legal provisions restricting the right to asylum. sustained autonomous interpretations of these norms that do not necessarily conform to those of supra-national bodies such as the European Court of Human Rights. 105 The Council has also limited its review of laws not yet promulgated by examining only the provisions specifically referred to it by political authorities. From its very first decisions, the Council has recognized its own competence to review all provisions of a deferred bill, beyond the specific issues raised by political authorities. 106 Yet, in most decisions reviewing immigration laws, the Council examines only those norms specifically at issue and leaves aside important questions regarding other norms. 107 Exceptions are rare. 108 While this problem is not specific to immigration laws, it is particularly problematic in this area of law because it involves fundamental rights issues. Until 2010, when the QPC preliminary ruling procedure was finally implemented, any provision not reviewed before its promulgation became exceedingly difficult to challenge later on. Many provisions that were problematic from a fundamental rights perspective were thereby permitted to evade scrutiny. Of course, these consequences invariably relate to public order considerations (i.e. maintaining immigration control measures), which are invoked to justify the continued violation of constitutional rights for several additional months.

IV. LOOSENING PROTECTION: THE EXAMPLE OF IMMIGRATION DETENTION

According to professor Serge Slama, the Constitutional Council's case law on immigration matters can be described as a belt loosened at every weight gain. 115 This is particularly true for immigration detention regimes, which have become an increasingly significant tool in French immigration policy over the past few decades. As the analysis in this Part IV will show, after an initial period of vigilance, during which the Council rejected longer detention periods, the Council gradually weakened its standards and began to accept longer detention periods, from seven days initially to three months today. It has become quite clear that the Council is no longer inclined to hinder in any way the legislator's policy priority of 'fighting against irregular migration'. The example of immigration detention thus demonstrates the Council's willingness to reinforce stricter policy preferences by translating them into constitutional language. In particular, this analysis will show that policy considerations, while not explicitly acknowledged in the Council's reasoning, played an overwhelming role in the Council's decision to overturn its own case law.

Initial Vigilance: Rejecting Longer Detention

In its early decisions regarding immigration laws, the Council was particularly vigilant when it came to immigration detention regimes. Article 66 of the 1958 Constitution asserts the right to be free from arbitrary detention and septembre 2019, Mme Alaitz A. et autre, cons 11 (my translation). But see Cons const, décision n° 2018-709 QPC du 1 juin 2018, Section française de l'observatoire international des prisons et autres, cons 12. entrusts the judiciary authority with protecting individual liberty. While the Council always accepted in principle the possibility of administrative detention to enforce deportation measures, 116 it was initially determined to enforce relatively strict criteria for judicial review and limits on duration. The Council did not hesitate to strike down provisions on these grounds in four different decisions between 1980 and 1993.

In a 1980 decision, the Council first established that juge judiciaire (judicial branch as opposed to administrative judges) must intervene as early as possible to review any decision imposing retention administrative (administrative detention) or extending its duration for an additional period. 117 The Council accepted one detention regime that provided for judicial intervention after forty-eight hours of administrative detention, but rejected another regime in which such intervention was not envisaged for more than seven days (either automatically or upon the detainee's request). In a 1986 decision, the Council examined and struck down a detention provision on its own initiative, even though members of Parliament had not criticized it in their brief to the Council. The relevant law provided the possibility of a six-day extension of detention, followed by a second extension of three additional days, should the administration demonstrate that it encountered 'particular difficulties hindering the detainee's deportation'. The Council deemed that the second extension, bringing the total duration of detention to ten days, could only be granted in cases of 'absolute emergency and particularly serious threats to public order'. 120 It deemed the law in question too broad to satisfy these constitutional requirements, as it was not limited to such exceptional cases.

In a 1992 decision, the Council rejected a provision establishing transit zones for foreigners whose entry in France was refused and for those requesting asylum at the border. It first confirmed that detaining foreigners at the border was a lesser restriction of individual liberty than detaining individuals 116 Cons const, décision n° 79-109 (n 67). already on French soil. 121 Although the Council did not explicitly justify this position, it seems to have been swayed by the fact that the law allowed foreigners detained at the border to leave the transit zone at any time by leaving France for a foreign destination where they could be admitted. 122 Nonetheless, given the degree of constraint and the duration of the measure, the Council still insisted that review by a juge judiciaire must be made available as early as possible, and that in any case the maximum duration of the measure had to be 'reasonable'. 123 The Council deemed that these constitutional requirements were not respected under the transit zone regime, which authorized the detention of foreigners for up to twenty days without any judicial review, subject to extension for an additional ten days by ruling of an administrative judge (not a juge judiciaire).

Finally, in the notable August 1993 decision cited multiple times above, the Council reiterated the rule it had established in 1986. The legislator had again attempted to authorize a second extension of administrative detention, for a cumulative total of ten days, in cases where 'the foreigner did not provide the competent authority with a travel document enabling the execution of a [deportation measure]'. The Council struck down the provision, maintaining its position that a second extension of the detention measure could only be allowed in exceptional cases of absolute emergency and particularly serious threats to public order. 124

Later Shift: Accepting Longer Detention

A shift started to occur in the late 1990s, as the legislator and the administration continued to harshen their immigration policies. In 1997 and 1998, the Council failed to examine provisions extending the scope of cases in which a second extension of a term of detention could be granted beyond seven days. In 1997, a law was brought before the Council that maintained measures originally taken in a December 1993 law that was not referred to the 121 Cons const, décision n° 92-307 DC (n 106) cons 13-14. Council. 125 These measures violated the limits set by the Council in its and 1993 decisions, but the Council nevertheless declined to consider the relevant provisions. 126 In 1998, another law came before the Council that increased the length of the second extension to five days, thus bringing the total duration of detention to twelve days. It also extended again the scope of cases where a second extension could be requested. Yet the Council did not react, 127 signaling what would later be deemed tacit approval. 128 Then, in a 2003 decision, the Council explicitly abandoned its earlier case law. It accepted a total duration of thirty-two days of detention, with two extensions of fifteen days after an initial forty-eight hours. The Council also relaxed the restrictions on cases in which a second extension could be granted. Indeed, the Council accepted the new regime wholesale, subject to the réserve d'interprétation that the juge judiciaire may interrupt detention at any time, either on its own initiative or upon the detainee's request. 129 The Council's ruling lifted virtually all of the previously applicable restrictions and essentially afforded the legislator the opportunity to extend the maximum length of immigration detention at will. Indeed, in 2011, the Council authorized an extension of the maximum term of detention to forty-five days, referring explicitly to the objective of fighting irregular migration as part of the safeguarding of public order, a recognized objectif à valeur constitutionnelle. Its approval was subject to the same réserve d'interprétation it had formulated in 2003, 130 but this time the juge judiciaire was not required to intervene until five days into the term of detention (or, in cases where a foreigner was placed in custody prior to an order of administrative detention, up to seven). 131 The Council deemed that the legislator's decision that an administrative judge should intervene prior to the juge judiciaire served the interests of the good administration of justice, which has also been recognized as an objectif à valeur constitutionnelle. 132 One begins to wonder how this respects the Council's own requirement, established in 1980, that the juge judiciare must intervene as early as possible. 133 Finally, in 2018, the Council accepted a maximum length of detention of ninety days, under the same reasoning and subject to the same réserve d'interprétation as in 2003 and 2011. 134 Although the Council pronounced the measure 'adequate, necessary and proportionate', its reasoning in reaching this conclusion remained superficial and abstract. It failed to respond to concrete arguments set forth by civil society, national human rights institutions and members of Parliament as to the inefficiency of long-term detention and its negative psychological impact on detainees. 135 Unfortunately, some of these concerns have materialized since the law entered into force. 136 The dramatic shift in the Council's case law from 2003 onwards was clearly a deliberate effort to provide a wider margin of action for the legislator in the 'fight against irregular migration'. Although the 2003 decision itself offers only limited and formalistic reasons as to why the Council departed from its previous decisions, the official commentary of the decision published on the institution's website explicitly acknowledges this reversal, essentially relying on the arguments presented by the Government in its written observations. A communication from former President of the Constitutional Council Pierre Mazeaud in 2005 also offers precious insights. He himself labels the 2003 decision a 'revirement de jurisprudence' (reversal of case law) grounded on 'considerations of a non-legal nature'. 138 Mazeaud explains:

A 13 August 1993 decision deemed that, except in cases of particularly serious threats to public order, the Constitution prohibited the detention of foreigners targeted by deportation measures for more than a week.

A decision of 20 November 2003 nevertheless validated provisions which, to provide the administration with sufficient time to enforce deportation measures in an efficient manner, increased the maximum duration of detention to thirty-two days. Realism played an important role in this decision. The fact that almost all European countries implemented a higher maximum duration could only influence the Council and push for a reversal. Another determinant factor was the legislator's will to solve a situation where only a low proportion of deportation measures were enforced, which public opinion struggled to accept. 139 It is striking to observe how much the language of 'realism' used by the former President of the Council resonates with the Government's observations in the proceedings leading to an April 1997 decision reviewing an immigration law. 140 It is, of course, not so surprising if one considers the lengthy political career Mazeaud enjoyed before becoming a member of the Council. Many other members of the Council have had similar career paths, which helps observed in many other areas of law. In so doing, it has recognized extensive police powers for the legislator, setting aside criminal law guarantees and favoring public order considerations over rights. Taking the example of immigration detention, after an initial period of vigilance during which the Council rejected longer detention periods, the Council gradually weakened its standards and allowed the legislator to lengthen terms from an initial seven days to three months today. The analysis in this article highlighted the Council's willingness and ability to adapt its case-law to support more restrictive policy preferences. It also demonstrated that policy considerations, while not explicitly acknowledged as such in the Council's case law, played an overwhelming role in the Council's decision to depart from its own established case law.

Overall, over the past few decades, the French Constitutional Council has remained faithful to the immigration policy preferences pursued by the legislator and the Government. The Council's most recent case law is particularly striking in this regard. As the 'fight against irregular migration' became an overarching goal of French and European immigration policies, the Council translated this aim into constitutional terms by linking it to the safeguard of public order, a recognized objectif à valeur constitutionnelle against which foreigners' rights -even the most fundamental -are often unfavorably balanced. The Council has thus deliberately abandoned a rights-based approach to immigration matters to facilitate stricter immigration control.

4

  Andreas Fahrmeir, Olivier Faron and Patrick Weil (eds), Migration Control in the North Atlantic World: The Evolution of State Practices in Europe and the United States from the French Revolution to the Inter-War Period (Berghahn Books 2002).

  5

6

  Gérard Noiriel, 'Surveiller les déplacements ou identifier les personnes ? Contribution à l'histoire du passeport en France de la I ère à la III e République' (1998) 30 Genèses 77. 7 Gérard Noiriel, Le Creuset français. Histoire de l'immigration. XIX e -XX e Siècle (Editions du Seuil 2006), 71; Laurent Dornel, La France hostile. Socio-histoire de la xénophobie (1870-1914) (Hachette Littératures 2004).

9

  Décret du 2 octobre 1888 relatif aux étrangers résidant en France. See also loi du 8 août 1893 relative au séjour des étrangers en France et à la protection du travail national. 10 Décret du 2 avril 1917 portant création d'une carte d'identité à l'usage des étrangers. 11 Décret-loi du 2 mai 1938 sur la police des étrangers. 12 Ordonnance du 2 novembre 1945 relative à l'entrée et au séjour des étrangers en France. 13 Danièle Lochak, 'Les politiques de l'immigration au prisme de la législation sur les étrangers' in Didier Fassin, Alain Morice and Catherine Quiminal (eds), Les Lois de l'inhospitalité. Les politiques de l'immigration à l'épreuve des sans-papiers (La Découverte 1997) 31-32; Catherine Wihtol de Wenden, 'Ouverture et fermeture de la France aux étrangers. Un siècle d'évolution' (2002) 73 Vingtième Siècle. Revue d'histoire 33.

39'

  Any man persecuted in virtue of his actions in favor of liberty may claim the right of asylum upon the territories of the Republic'.

  40

41

  For a detailed analysis, see Danièle Lochak, 'L'étranger et les droits de l'homme' in Service public et libertés : mélanges offerts au professeur Robert-Édouard Charlier (Editions de l'Université et de l'enseignement moderne 1981) 615-633; Danièle Lochak, 'La citoyenneté : un concept juridique flou' in Dominique Colas, Claude Emeri and Jacques Zylberberg (eds), Citoyenneté et nationalité. Perspectives en France et au Québec (Presses Universitaires de France 1991) 179-207; Kissangoula (n 3). 42 Since 1789, France has known sixteen different constitutions. 43 Article 3 paragraph 1 of the 1958 Constitution. See also article 3 of the 1789 Declaration on the Rights of the Man and of the Citizen, which provides that 'the principle of any Sovereignty lies essentially in the Nation. No corporate body, no individual may exercise authority that does not expressly emanate from it'. 44 Article 3 paragraph 4 of the 1958 Constitution: 'All French nationals of either sex who have reached their majority and are in possession of their civil and political rights may vote as provided for by statute.' Emphasis added. I have slightly modified the English translation provided by the Constitutional Council -which uses the term 'citizens' -to reflect the nuance between the terms nationals and citizens.

  translation). He mentions the example of article 34 of the 1958 Constitution, according to which 'statutes shall determine the rules concerning: -civic rights and the fundamental guarantees granted to citizens for the exercise of their civil liberties […]'. Ibid (my translation, emphasis in original). 52 Labayle (n 46) 48 (my translation).

  this move const, décision n° 2017-674 QPC du 1 décembre 2017, M. Kamel D., cons 4; Cons const, décision n° 2018-762 DC du 15 mars 2018, Loi permettant une bonne application du régime d'asile européen, cons 9; Cons const, décision n° 2018-717/718 QPC du 6 juillet 2018, M. Cédric H. et autre, cons 9; Cons const, décision n° 2018-770 DC du 6 septembre 2018, Loi pour une immigration maîtrisée, un droit d'asile effectif et une intégration réussie, cons 87. 60 Cons const, décision n° 93-325 DC (n 54) cons 2 (my translation). For an earlier assertion of this principle, see Cons const, décision n° 89-266 DC (n 55) cons 6. It seems the Council was initially merely describing the regime established by the ordonnance du 2 novembre 1945 relative à l'entrée et au séjour des étrangers en France before the current Constitution was even adopted. In subsequent decisions, the assertion nevertheless became autonomous and appears to have become an implicit principle of French constitutional law. 61 Cons const, décision n° 2011-631 DC (n 59) cons 64; Cons const, décision n° 2017-674 QPC (n 59) cons 4; Cons const, décision n° 2018-762 DC (n 59) cons 9; Cons const, décision n° 2018-770 DC (n 59) cons 87. 62 Cons const, décision n° 97-389 DC (n 59) cons 24 (my translation). 63 Fong Yue Ting v UnitedStates, 149 US 698, 730 (1893). Three justices issued dissenting opinions in which they expressed their 'utter' disagreement.

64

  Cons const, décision n° 89-261 DC (n 22) cons 12; Cons const, décision n° 93-325 DC (n 54) cons 7. 65 See inter alia Cons const, décision n° 96-377 DC du 16 juillet 1996, Loi tendant à renforcer la répression du terrorisme et des atteintes aux personnes dépositaires de l'autorité publique ou chargées d'une mission de service public et comportant des dispositions relatives à la police judiciaire, cons 11; Cons const, décision n° 98-399 DC du 5 mai 1998, Loi relative à l'entrée et au séjour des étrangers en France et au droit d'asile, cons 7.

  66

67

  Cons const, décision n° 79-109 DC du 9 janvier 1980, Loi relative à la prévention de l'immigration clandestine et portant modification de l'ordonnance n° 45-2658 du 2 novembre 1945 […], cons 6 (my translation).

  innocence principle, guaranteed by article 9 of the Declaration of the Rights of Man and of the Citizen.

79

  Cons const, décision n° 97-389 DC (n 59) cons 43-45; Cons const, décision n° 2017-674 QPC (n 59) cons 10. 80 On axiological hierarchies, see Véronique Champeil-Desplats, Théorie générale des droits et libertés. Perspective analytique (Dalloz 2019) 305-330; Riccardo Guastini, Teoría analítica del derecho (Zela 2017) 119-122. 81 For instance, the Council has relied on article 34 of the Constitution, which makes no explicit reference to public order. 82 Cons const, décision n° 2006-539 DC (n 59) cons 11-14. 83 Cons const, décision n° 2003-484 DC (n 59) cons 23; Cons const, décision n° 2011-631 DC (n 59) cons 64; Cons const, décision n° 2018-717/718 QPC (n 59) cons 9; Cons const, décision n° 2019-797 QPC (n 77) (my translation).

  89 84 See the Government's observations lodged before the Council in Cons const, decision n° 97-389 DC (n 59) (my translation). 85 Nathalie Ferré, 'La production de l'irrégularité', in Fassin, Morice and Quiminal (n 13), 47-64 (my translation). 86 Brief of MPs lodged before the Constitutional Council on 27 March 1997. 87 Arthur Dyevre, 'The French Constitutional Council' in András Jakab, Arthur Dyevre and Giulo Itzcovich (eds), Comparative Constitutional Reasoning (Cambridge University Press 2017), 323-355. 88 See e.g. Cons const, décision n° 2011-631 DC (n 59) cons 79. 89 See inter alia Cons const, décision n° 2018-762 DC (n 59) cons 9-16; Cons const, décision n° 2018-770 DC (n 59) cons 70-76; Cons const, décision n° 2011-631 DC (n 59) cons 66, 72. Although in recent years, the Council has begun employing

94

  Cons const, décision n° 89-261 DC (n 22) cons 15; Cons const, décision n° 2011-217 of 3 février 2012, M. Mohammed Akli B., cons 4; Cons const, décision n° 2015-501 QPC du 27 novembre 2015, M. Anis T., cons 8; Cons const, décision n° 2017-674 QPC (n 59) cons 25; Cons const, décision n° 2018-717/718 QPC (n 59) cons 18. 95 Cons const, décision n° 89-261 DC (n 22). 96 Cons const, décision n° 83-165 DC du 20 janvier 1984, Loi relative à l'enseignement supérieur, cons 42; Cons const, décision n° 84-181 DC du 11 octobre 1984, Loi visant à limiter la concentration et à assurer la transparence financière et le pluralisme des entreprises de presse, cons 37; Cons const, décision n° 93-325 DC (n 54) cons 81; Cons const, décision n° 94-345 DC du 29 juillet 1994, Loi relative à l'emploi de la langue française, cons 5. 97 Cons const, décision n° 2002-461 DC du 29 août 2002, Loi d'orientation et de programmation pour la justice, cons 67; Const. const., décision n° 86-210 DC du 29 juillet 1986, Loi portant réforme du régime juridique de la presse, cons 2. For developments on how the 'ratchet theory' was abandoned, see Véronique Champeil-Desplats, 'Le Conseil constitutionnel a-t-il une conception des libertés publiques ?' (2012) 7 Jus Politicum 15-17.

99

  Cons const, décision n° 86-216 DC (n 75) cons 14. 100 Cons const, décision n° 89-261 DC (n 22) cons 15; Cons const, décision n° 93-325 DC (n 54) cons 2; Cons const, décision n° 2011-631 DC (n 59) cons 67 ; Cons const, décision n° 2018-770 DC (n 59) cons 71. 101 Cons const, décision n° 74-54 DC (n 91) cons 1-7. Article 55 of the Constitution provides that 'treaties or agreements duly ratified or approved shall, upon publication, prevail over Acts of Parliament, subject, with respect to each agreement or treaty, to its application by the other party'. Given that article 61 provides that the Constitutional Council 'shall rule on [the] conformity of [referred bills] to the Constitution', one could argue that the Council should review the conformity of referred laws to international treaties so as to ensure respect for article 55 of the Constitution. 102 Cons const, décision n° 93-325 DC (n 54) cons 2; Cons const, décision n° 97-389 DC (n 59) cons 13. 103 Cons const, décision n° 98-399 DC (n 65) cons 11-12. 104 Cons const, décision n° 2018-770 DC (n 59) cons 54.

115

  Serge Slama, 'La rétention des « Dublinables » : le Conseil Constitutionnel admet une rétention préventive sans perspective immédiate d'éloignement' (2018) Lexbase, La lettre juridique n° 739 <https://www.lexbase.fr/revues-juridiques/ 45196432-document-elastique> accessed 3 September 2021.

  décision n° 86-216 DC (n 75) cons 21 (my translation). 120 Ibid cons 22 (my translation).

122

  This point was explicitly debated during the deliberation session. See minutes of the Council's deliberation session of 24 and 25 February 1992. 123 Cons const, décision n° 92-307 DC (n 106) cons 16 (my translation). 124 Cons const, décision n° 93-325 DC (n 54) cons 100 (my translation).

  décision n° 2003-484 DC (n 59) cons 66. 130 Cons const, décision n° 2011-631 DC (n 59) cons 75. 131 Ibid cons 73.

  'La place des considérations extra-juridiques dans l'exercice du contrôle de constitutionnalité. EREVAN : 29 septembre -2 octobre 2005' (Conseil constitutionnel, 2005) <https://www.conseil-constitutionnel.fr/sites/ default/files/as/root/bank_mm/pdf/Conseil/20051001erevan.pdf> accessed January 2021, 13-14 (my translation). 139 Ibid 14 (my translation). 140 Government's observations before the Council in Cons const, décision n° 97-389 DC (n 59).

  In 1836, the Council of State refused to review the legality of administrative acts of deportation and detention, deeming these acts 'high police' powers of the French administration, immune from judicial review. CE, 2 août 1836, Naundorff, n° 12843, Rec p 379. It was only very gradually that the Council of State asserted authority to review such acts. At first, only questions of administrative procedure were open to challenge. See Stéphane Duroy, 'Le contrôle juridictionnel des mesures de police relatives aux étrangers sous la Troisième République' in Marie-Claude Blanc-Chaléard and others (eds), Police et migrants : France 1667-1939 (Presses Universitaires de Rennes 2001) 91-104. For a general overview of issues regarding the fundamental rights of foreigners in France, see Le Défenseur des droits, Les droits fondamentaux des étrangers en France (mai 2016) <https://www.defenseurdesdroits.fr/sites/default/files/atoms/files/ 736160170_ddd_rapport_droits_etrangers.pdf> accessed 15 January 2021.

	17	For a brief historical account of the evolution of immigration law between 1945
		and 2011, see Thomas Ribémont, Introduction au droit des étrangers en France (De
		Boeck 2012) 12-20.

18

  While the Council of State remains significant in reviewing the legality of administrative acts in the field of immigration, 23 the Constitutional Council has undoubtedly become a new battleground for politically charged challenges to French immigration

	19	For a critical account of this trend, see Karine Parrot, Carte blanche. L'Etat contre
		les étrangers (La Fabrique 2019).
	20	Danièle Lochak, 'L'intégration comme injonction. Enjeux idéologiques et
		politiques liés à l'immigration' (2006) 64 Cultures & Conflits 131.
	21	In 2020, there were around 5.1 million foreigners in France, thus amounting to 7.6
		per cent of the total population of France (67 million inhabitants). This
		proportion has varied significantly over time. It went from 6.6 percent in 1931 to
		4.1 per cent in 1954. It then peaked at 6.8 per cent in 1982 before decreasing to 5.5
		per cent in 1999. It has increased again during the past two decades. 'L'essentiel
		sur… les immigrés et les étrangers' (Institut national de la statistique et des études
		économiques, 1 July 2021) <https://www.insee.fr/fr/statistiques/3633212> accessed
		29 October 2021.
	22	In some rare cases, members of Parliament have argued that the law in question
		granted excessive rights to foreigners and did not protect French nationals
		sufficiently. See in particular Cons const, décision n° 89-261 DC du 28 juillet 1989,
		Loi relative aux conditions de séjour et d'entrée des étrangers en France; Cons const,
		décision n° 91-294 DC du 25 juillet 1991, Loi autorisant l'approbation de la convention
		d'application de l'accord de Schengen…. The French Parliament is composed of two
		chambers: the Assemblée nationale (National Assembly) and the Sénat (Senate).
		Members of the National Assembly are called députés (MPs).
	23	The Council of State also reviews the conformity of administrative acts to the
		European Convention of Human Rights, in particular article 8 which safeguards
		the right to respect for private and family life. CE, Ass, 19 avril 1991, M. Belgacem,
		n° 107470, Rec p 152; CE, Ass, 19 avril 1991, Mme Babas, n° 117680, Rec p 162.

  Second, 30 Cons const, décision n° 71-44 DC du 16 juillet 1971, Loi complétant les dispositions des articles 5 et 7 de la loi du 1 er juillet 1901 relative au contrat d'association. Loi constitutionnelle n° 74-904 du 29 octobre 1974 portant révision de l'article 61 de la Constitution.

	33	Articles 29 and 30 of loi constitutionnelle n° 2008-724 du 23 juillet 2008 de modernisation
		des institutions de la Ve République.

31

Louis Favoreu and others, Droit constitutionnel (Dalloz 2018) 136. 32

  Loi relative aux conditions d'entrée et de séjour des étrangers en France, cons 18. For an analysis of the Council of State's influence, see Kissangoula (n 3) 200-205.

		while only
	74	CE, Ass, 8 décembre 1978, GISTI, CFDT et CGT, n° 10097, 10677, 10679, Rec p
		493.
	75 Cons const, décision n° 86-216 DC du 3 septembre 1986, 76 This includes six instances in which no explicit mention was made of conciliation,
		but where public order considerations were clearly invoked (sometimes through
		other notions such as public interest or administrative police powers) to restrict
		constitutional rights. Cons const, décision n° 93-325 DC (n 54) cons 25, 60, 87;
		Cons const, décision n° 97-389 DC (n 59) cons 24, 52; Cons const, décision n° 2003-
		485 DC du 4 décembre 2003, Loi modifiant la loi n° 52-893 du 25 juillet 1952 relative
		au droit d'asile, cons 56.
	77	Cons const, décision n° 86-216 DC (n 75) cons 18; Cons const, décision n° 93-325
		DC (n 54) cons 19-22, 25, 56, 60, 63, 87; Cons const, décision n° 97-389 DC (n 59)
		cons 10-14, 21, 24, 36, 71-72; Cons const, décision n° 2003-467 DC (n 59) cons 110;
		Cons const, décision n° 2003-484 DC (n 59) cons 23, 38-39, 57; Cons const,
		décision n° 2003-485 DC (n 76) cons 56; Cons const, décision n° 2005-528 DC
		(n 59) cons 14, 16; Cons const, décision n° 2006-539 DC (n 59) cons 13-14; Cons
		const, décision n° 2007-557 DC du 15 novembre 2007, Loi relative à la maîtrise de
		l'immigration, à l'intégration et à l'asile, cons 11; Cons const, décision n° 2011-631 DC
		(n 59) cons 72, 78-79; Cons const, décision n° 2016-580 QPC du 5 octobre 2016, M.
		Nabil F., cons 12; Cons const, décision n° 2018-762 DC (n 59) cons 9-16; Cons
		const, décision n° 2018-770 DC (n 59) cons 63, 92, 99; Cons const, décision n° 2019-
		797 QPC du 26 juillet 2019, Unicef France et autres, cons 11.
	78	Cons const, décision n° 97-389 DC (n 59) cons 52; Cons const, décision n° 2017-
		674 QPC (n 59) cons 11-12; Cons const, décision n° 2018-770 DC (n 59) cons 76.

  Although the Council decided in a 1985 case (unrelated to immigration) that it could examine already promulgated provisions if they

	105	Ibid.
	106	Cons const, décision n° 60-8 DC du 11 août 1960, Loi de finances rectificative pour
		1960, cons 5. This position was explicitly confirmed in a case regarding an
		immigration law in 1992. Cons const, décision n° 92-307 DC du 25 février 1992, Loi
		modifiant l'ordonnance n° 45-2658 du 2 novembre 1945 [...], cons 1.
	107	Cons const, décision n° 79-109 DC (n 67); Cons const, décision n° 89-261 DC (n
		22) cons 32; Cons const, décision n° 89-266 DC (n 55) cons 9; Cons const, décision
		n° 93-325 DC (n 54) cons 134; Cons const, décision n° 97-389 DC (n 59) cons 77;
		Cons const, décision n° 98-399 DC (n 65) cons 21; Cons const, décision n° 2003-
		485 DC (n 76) cons 65; Cons const, décision n° 2006-539 DC (n 59) 30; Cons const,
		décision n° 2007-557 DC (n 77) cons 30; Cons const, décision n° 2011-631 DC (n 59)
		cons 96; Cons const, décision n° 2016-728 DC du 3 mars 2016, Loi relative au droit
		des étrangers en France, cons 7; Cons const, décision n° 2018-762 DC (n 59) cons 24.
	108	But see Cons const, décision n° 86-216 DC (n 75) cons 21-22; Cons const, décision
		n° 92-307 DC (n 106) cons 35-37; Cons const, décision n° 2003-484 DC (n 59) cons
		98-101; Cons const, décision n° 2018-770 DC (n 59) cons 115-118.

European Journal of Legal Studies {Special Issue

See e.g. circulaire n° 9-74 du 5 juillet 1974 du secrétaire d'Etat auprès du ministre du travail, relative à l'arrêt provisoire de l'introduction de travailleurs étrangers; circulaires n° 11-74 du 9 juillet 1974, n° 17-74 du 9 août 1974 et n° 22-74 du 27 décembre 1974 du secrétaire d'Etat auprès du ministre du travail, suspendant provisoirement l'introduction en France des familles des travailleurs étrangers; circulaire n° 77-280 du 20 juin 1977 relative à l'application de 'l'aide au retour'.

CE, Ass, 8 décembre 1978, GISTI, CFDT et CGT, n° 10097, 10677, 10679, Rec p 493.

Francis Hamon and Michel Troper, Droit constitutionnel (Librairie générale de droit et de jurisprudence 2020) 804.

Ibid 806.

Ibid 802.

Article 61 of the 1958 Constitution.

Loi organique n° 2009-1523 du 10 décembre 2009 relative à l'application de l'article 61-1 de la Constitution.

Article 61-1 of the Constitution does not set any condition of nationality.

Questions of legislative procedure, amongst others, thus remain out of reach for mere litigants.

Ibid 255 (my translation).

Ibid cons 72.

Admittedly, the Council did push back on part of the legislator's harsh immigration control agenda in its 2011 decision, striking down part of a provision authorizing the long-term detention of foreigners subject to an interdiction du territoire (re-entry ban) for terrorist acts or expulsion for behavior linked to terrorist activities. The Council found that this regime violated the right to individual liberty by allowing a twelve-month extension after an initial six-month period of detention. However, in doing so, it implicitly confirmed the constitutionality of the initial six-month period. Ibid cons 76.

Exercising Traditional Self-Restraint

French legal culture has traditionally been hostile to judicial power. Even with the expansion of its role, the Council has remained careful to appear deferential to Parliament so as to preserve its institutional legitimacy. This is true even when it comes to the protection of fundamental rights, an area in which the Constitutional Council's case law has not escaped criticism. 90 Indeed, the Council has demonstrated self-restraint in most areas of law. Since 1975, it has stated many times that the Constitution 'does not grant the Constitutional Council a general power of appreciation and decision identical to that of Parliament, it merely gives it competence to review the conformity of challenged laws to the Constitution'. 91 Quite significantly, this very statement has been invoked by the Constitutional Council to uphold diametrically opposed laws one after the other. In 2011, the Council confirmed the constitutionality of a set of provisions interpreted as restricting marriage to heterosexual couples. 92 Only two years later, the Council reviewed a legal reform opening marriage to same-sex couples and found it constitutional on the very same grounds. 93 In both cases, Councillors wrote that it was for the legislator, not the Council, to decide what sort of "different situation" could justify "different treatment" without violating the principle of equality. This allowed the Council to avoid any discussion of the substance of the matter. Decisions reviewing immigration laws tend to follow a similar approach. In fact, the Council has Michael Koskas, 'Le dynamisme de la proportionnalité : enjeux de la fragmentation tripartite du principe dans le processus juridictionnel' (2019) 15 Revue des Droits de l'Homme s 42. were modified, completed or affected by a new law, 109 it does not seem to have made much use of this power in decisions reviewing immigration laws. 110 Lastly, the Council has attenuated the impact of its decisions in two ways. The first is the aforementioned 'réserves d'interprétation', which allow the Council to maintain deference to the legislator by upholding questionable provisions under particular interpretations that the Council provides. This approach has clear limitations. First, when dealing with laws that are not yet promulgated, the Council may encounter difficulties in anticipating how a provision will be enforced. 111 Second, the Council's interpretations are sometimes quite vague, leaving plenty of room for subsequent interpretations by the administration and judges. 112 Furthermore, some of the Council's interpretations appear to afford rather weak protection against potential violations of foreigners' rights. 113 Lastly, there is no direct mechanism ensuring that the Council's interpretations will be faithfully applied by the administration and judges.

The second way in which the Council has attenuated the impact of its decisions -specifically preliminary rulings under the QPC procedure -is through modulation des effets dans le temps (postponing their effectiveness to a later date). By such means, the Council can strike down a provision but specify a grace period of several months before the censored provision will actually be abrogated. The rationale for this remedy is that immediate abrogation would in some cases have 'manifestly excessive consequences'. explain the institution's deferential approach vis-à-vis the legislator, as well as its shared policy perspectives. 141 The Council's endorsement of stricter migration policies can be observed in several other recent decisions, which have confirmed the expanded use of immigration detention. In a June 2011 decision, the Council approved the creation of temporary border detention sites (outside of border crossing points) that do not offer the same guarantees as zones d'attente (waiting zones). 142 In a March 2018 decision, the Council also refused to strike down a provision which authorized the preventive detention of asylum seekers who are likely to be transferred to another member state of the European Union under the Dublin Regulation. 143 Last, but not least, in a September 2018 decision, the Council deemed constitutional the detention of children 'accompanying' adults targeted by deportation measures. 144 V. CONCLUDING REMARKS For the most part, the French Constitutional Council has endorsed the immigration policy preferences of the legislator and the Government by translating them into constitutional language. This article has shown that the Council's seemingly neutral methods of reasoning are in fact politically oriented instruments providing stable support for restrictive immigration policy preferences. The Council enjoys a rather wide margin of action on immigration matters, as it is not substantially constrained by the Constitution itself in this area. Nonetheless, it has exercised considerable self-restraint on such questions, employing permissive methods of reasoning 141 This is particularly manifest in the minutes of deliberation sessions of the Council, which are published twenty-five years after the publication of the decision itself. Cons const, décision n° 2018-762 DC (n 59) cons 16. The measure was unprecedented, as immigration detention had always been used to enforce a deportation measure already taken by the administration in cases where there was a reasonable perspective that the foreigner would effectively be deported. 144 Cons const, décision n° 2018-770 DC (n 59) cons 64 (my translation). The Council considered that it was in the child's interest not to be separated from the adult she 'accompanied' and that her interest in not being detained could be balanced against public order considerations favoring the adult's effective deportation.


François Diakhaté

Marc Perache

Raymond Namyst

Herve Jourdren

Efficient shared memory message passing for inter-VM communications

Thanks to recent advances in virtualization technologies, it is now possible to benefit from the flexibility brought by virtual machines at little cost in terms of CPU performance. However on HPC clusters some overheads remain which prevent widespread usage of virtualization. In this article, we tackle the issue of inter-VM MPI communications when VMs are located on the same physical machine. To achieve this we introduce a virtual device which provides a simple message passing API to the guest OS. This interface can then be used to implement an efficient MPI library for virtual machines. The use of a virtual device makes our solution easily portable across multiple guest operating systems since it only requires a small driver to be written for this device. We present an implementation based on Linux, the KVM hypervisor and Qemu as its userspace device emulator. Our implementation achieves near native performance in terms of MPI latency and bandwidth.

Introduction

Thanks to their excellent isolation and fault tolerance capabilities, virtual machines (VM) have been widely embraced as a way to consolidate network servers and ease their administration. However, virtual machines have not yet been adopted in the context of high performance computing (HPC), mostly because they were incurring a substantial overhead. Recently, advances in hardware and software virtualization support on commodity computers have addressed some of these performance issues. This has led to many studies of how HPC could take advantage of VMs [START_REF] Mergen | Virtualization for highperformance computing[END_REF].

First, HPC clusters too would greatly benefit from the ease of management brought by virtual machines. Checkpoint/restart and live migration capabilities could provide fault tolerance and load balancing transparently to applications. Moreover, VMs give cluster users a greater control of their software environment without involving system administrators.

Second, it should be possible to use VMs to improve the performance of HPC applications. Using a minimal hypervisor and guest operating system can decrease system noise and therefore greatly increase performance of some communication operations [START_REF] Hudson | Network performance impact of a lightweight linux for cray xt3 compute nodes[END_REF]. More generally, virtualization allows the use of specialized guest OSes with scheduling or memory management policies tuned for a specific application class.

Nonetheless, whatever VMs are used for, their integration within HPC environments must have a negligible performance overhead to be successful. Since MPI (Message Passing Interface) is the most widely spread communication interface for compute clusters, designing efficient MPI implementations in the context of virtual machines is critical. Some high performance Network Interface Cards can be made accessible directly from within VMs, thus providing near native communication performance when VMs are hosted on different physical machines [START_REF] Huang | Virtual Machine Aware Communication Libraries for High Performance Computing[END_REF]. However, implementing fast communication between virtual machines over shared memory is also crucial considering the emergence of multicore cluster nodes.

Indeed, to execute MPI applications, it is desirable to use monoprocessor VMs and one MPI task per virtual machine: it allows finer-grained load balancing by VM migration and multiprocessor VMs exhibit additional overheads due to interferences between host and guest schedulers [START_REF] Ranadive | Performance implications of virtualizing multicore cluster machines[END_REF][START_REF] Uhlig | Towards scalable multiprocessor virtual machines[END_REF]. Thus, one major challenge is to implement efficient message passing between processes running inside separate VMs on shared memory architectures.

In this paper, we present a new mechanism to perform efficient message passing between processes running inside separate VMs on shared memory architectures. The main idea is to provide a virtual message passing device that exposes a simple yet powerful interface to the guest MPI library. Our approach enables portability of the guest MPI implementation across multiple virtualization platforms, so that all the complex code dealing with optimizing memory transfers on specific architectures can be reused.

Fast MPI communication over shared-memory architectures

In recent years, shared memory processing machines have become increasingly prevalent and complex. On the one hand, the advent of multi-core chips has dramatically increased the number of cores that can be fitted onto a single motherboard. Intel announcement of an 80 core chip prototype shows that this trend is only going to intensify in the future. On the other hand the increase in the number of sockets per motherboard has led to the introduction of NUMA effects which have to be taken into account. As a result, many research efforts have been devoted to achieving efficient data transfers over these new architectures.

In this section, we give an insight on how to perform such data transfers both in a native context, that is between processes running on top of a regular operating system, and in a virtualized environment (i.e. between processes belonging to separate virtual machines).

Message passing on native operating systems

On a native operating system, data exchanges between processes can be performed in many different ways, depending on the size of messages. Nonetheless, all solutions rely only on two primitive mechanisms: using a two step copy through pre-allocated shared buffers or using a direct memory-to-memory transfer.

Shared memory buffers: Many MPI implementations set up shared memory buffers between processes at startup thanks to standard mechanisms provided by the OS (e.g. mmap, system V shared memory segments). Communication can then take place entirely in userspace by writing to and reading from these buffers. Several communication protocols have been proposed. In MVAPICH2 [START_REF] Chai | Designing high performance and scalable mpi intra-node communication support for clusters[END_REF] all processes have dedicated receive buffers for each peer process: no synchronization is required between concurrent sends to a single process, but n 2 buffers are required and the cost of polling for new messages dramatically increases with the number of processes. To alleviate these issues, Nemesis, one of MPICH2 communication channels [START_REF] Buntinas | Implementation and Shared-Memory Evaluation of MPICH2 over the Nemesis Communication Subsystem[END_REF], uses only one receive buffer per process. This requires less memory and ensures constant time polling with respect to the number of processes. However concurrent senders have to be synchronized, which can be done efficiently using lockless queues.

Buffer based communication makes an efficient usage of the shared caches found in multi-core chips because the buffers can be made small enough so that they fit in the cache. As a result, when communicating between two cores sharing a cache, the extra message copy induces very little overhead (Fig. 1). Moreover, high NUMA locality can be achieved by binding each process to a core and allocating their memory buffers on the closest memory node. Such implementations thus offer very low latencies. However, bandwidth usage is not optimal for large messages, especially when processes are executed on cores which do not share a cache level.

Direct transfer:

To avoid the extra copy induced by the use of a shared buffer, some MPI implementations aim at directly copying data between send and receive buffers and thus ensure maximum bandwidth usage for large messages. However, this requires that the copy be performed in a memory context where both send and receive buffers are mapped. That for, a first approach is to perform the copy within the operating system's kernel [START_REF] Jin | Limic: Support for high-performance mpi intra-node communication on linux cluster[END_REF]. This requires a dedicated kernel module which makes this solution less portable. Moreover each communication will incur system call or even page pinning costs that restrict usage of this technique to sufficiently large messages. For smaller messages, shared buffer based communication has to be used. Another approach is to use threads instead of processes to execute MPI tasks [START_REF] Demaine | A threads-only mpi implementation for the development of parallel programs[END_REF]. This allows a portable, userspace only implementation but requires that the application code be thread-safe. For example, it may not use global variables.

Message passing between virtual machines on the same host

The issues raised when trying to provide shared memory message passing between VMs are actually quite similar to those involved in the native case. Indeed, similarly to processes in an OS, VMs do not share the same virtual address space. Shared buffer (b) Separate caches Fig. 1. Performance impact of using a shared buffer: if communicating cores share their cache (a), the additional copy does not lead to a cache miss and therefore causes little overhead compared to a direct copy. On the other hand, if caches are separate (b), significant memory bandwidth is wasted.

Therefore, one of two things is necessary to allow communications between two VMs:

-Having a pool of shared physical pages mapped in both communicating VMs virtual address spaces, so that the VMs can communicate directly using any shared memory buffer message passing technique seen earlier with no additional overhead. This approach has been studied using Xen page sharing capability to provide fast socket [10] and MPI [START_REF] Huang | Virtual Machine Aware Communication Libraries for High Performance Computing[END_REF] communication. -Requesting a more privileged access to the memory of both VMs to perform the message transfers. This avoids unnecessary copies to a shared buffer but requires a costly transition to the hypervisor. This is once again a tradeoff between latency and bandwidth. To provide optimal performance, the most appropriate technique has to be picked dynamically depending on the size of the message to transmit. Additionally, VM isolation must be taken into consideration. A VM should not be able to read or corrupt another VM's memory through our communication interface, except for the specific communications buffers to which it has been granted access.

Designing a virtual message passing device

As we pointed out in section 2, two communication channels have to be provided to allow high speed MPI data transfers between VMs running on the same host: a low latency channel based on shared buffers accessible directly from guests' userspaces and a high bandwidth channel based on direct copies performed by the hypervisor.

To address the challenge of providing these two channels to guest OSes in a portable way, we introduce a virtual communication device which exposes a low-level, MPI-friendly interface.

Indeed, traditional operating systems expect to sit directly on top of hardware and to interact with it through various interfaces. Therefore, they already provide ways to deal with these interfaces and to export them to applications. As a result, to introduce guest OS support for a new device one usually only has to write a small kernel module or device driver. Moreover, most hypervisors already emulate several devices to provide basic functionality to VMs (network, block device, etc.) Thus, it is possible to emulate a new device without modifying the core of the hypervisor. A virtual device is therefore an easy and portable way to introduce an interface between hypervisors and guest operating systems.

An overview of the usage of these communication channels is provided in Figure 2. We now describe the key features of the device. Fig. 2. Overview of the virtual message passing device: shared memory buffers which can be mapped in userspace are provided for low latency communications (a,b,c). Send and receive requests can be posted so that the device performs direct memory copies between VMs (1,2,3,4)

Ports: All communication endpoints on a physical machine are uniquely identified by a port number. This allows several communication channels to be opened on each virtual machine to cater for most use cases. Several MPI processes on a single VM might typically want to use the device for communication with MPI processes running in other VMs. Using port numbers instead of VM identifiers allows to handle these cases seamlessly. A VM must open a port before it is able to issue requests originating from it and only one VM can have a given port opened at a time.

Shared memory:

The virtual device possesses onboard memory which corresponds to a shared memory buffer that can be used to communicate between VMs. More specifically, this memory is divided into blocks of equal size and each port is attributed a block. The actual communication protocol is unspecified and shall be implemented in the guest's userspace by the communication library (e.g. MPI library). This way, communications can be performed entirely in userspace and don't incur latency overheads due to context switching.

DMA transfers:

The virtual device can process DMA copy operations between arbitrary memory locations on all the VMs that use it. There are 2 types of requests: receive and send which both apply to an origin and a target port. They take two additional arguments:

-A list of pointers and sizes which describe a possibly scattered memory buffer to send to or receive from. -A completion register which is a pointer to an integer in the VM's memory.

Completion and failure of a request can be inferred by reading the specified integer. This allows to poll directly from the guest's userspace, thus reducing the number of transitions to the guest kernel and to the hypervisor.

The semantics of these operations are similar to MPI Irecv and MPI Isend semantics. In particular, they ensure that a VM can only write to an other VM's memory when and where it is authorized to do so.

Note that this interface is similar to those of high performance network cards which offer buffered and rendez-vous communication channels. This ensures that existing MPI libraries can be ported easily to support this virtual messaging device. While our solution still requires more porting work than a solution offering binary compatibility with the socket interface [10] it is also more efficient, if only because it doesn't incur the system call overhead induced by sockets.

Implementation

In this section we provide details on our preliminary implementation of this virtual device using Linux as both guest and host OS and the Kernel Virtual Machine hypervisor. We start by providing a brief overview of KVM and then proceed to describe the implementation of our device from the guest and host point of view.

The Kernel Virtual Machine

KVM is a Linux kernel module which allows Linux to act as an hypervisor thanks to hardware virtualization capabilities of newer commodity processors. It is thus capable of virtualizing unmodified guest OSes but also supports paravirtualization to optimize performance critical operations. It provides paravirtualized block and network devices and a paravirtualized MMU interface.

Using KVM, VMs are standard Linux processes which can be managed and monitored easily. A userspace component is used to perform various initialization tasks, among which allocating a VM's memory using standard allocation functions and launching it by performing an ioctl call on the KVM module (see Figure 3). This component is then asked to emulate the VM's devices: whenever the guest OS tries to access the emulated device, the corresponding instruction traps into the KVM module which forwards it to the userspace component so that the expected result may be emulated. A slightly modified version of QEMU is provided as KVM's default userspace component as it is capable of emulating many devices.

KVM VirtIO driver

LGuest VirIO driver Network driver

Message driver

Fig. 4. The VirtIO interface

As a result, it is interesting to note that in this model, implementing the emulation of a new virtual device does not introduce additional code into the host kernel. Everything is performed inside QEMU which is a userspace process.

Guest implementation

A Linux driver has been implemented to allow Linux guests to handle our device. In the following paragraphs, we describe how this driver accesses the device and how it exposes the virtual device's functions to userspace applications.

Accessing the device: To maximize portability, our device is accessed through the recently introduced VirtIO interface. This interface abstracts the hypervisor specific virtual device handling instructions into an hypervisor agnostic interface. As a result, as shown in Figure 4, the same drivers can be used to handle several hypervisors' implementations of a given device. Only one hypervisor specific driver is needed to implement the interface itself. The VirtIO interface is based on buffer descriptors, which are lists of pointers and sizes packed in an array. Operations include inserting these descriptors into a queue, signalling the hypervisor and checking if a buffer descriptor has been used.

In our virtual device, each open port is associated with a VirtIO queue through which DMA requests can be sent by queuing buffer descriptors. The first pointer/size pair of the buffer descriptor points to a header containing the parameters of the request: whether it is a send or a receive, its buffer size, its destination port and its completion register. The following pointer/size pairs describe the buffer to send or receive.

However there is no VirtIO interface for directly sharing memory between guests. Therefore the shared memory is exposed to guests as the onboard memory of a PCI device.

Userspace interface: Our Linux driver exposes the virtual device as a character device and uses file descriptors to define ports. A port is acquired with the open system call, which returns the corresponding file descriptor and released with close. Ioctl calls are used to issue DMA requests because they require custom arguments such as the destination port and completion register. Mmap allows to map the shared memory of the device in userspace.

Host implementation

The host is in charge of emulating the virtual device. It has to implement memory sharing and DMA copies between VMs. Using KVM, everything can be implemented inside the VMs' corresponding QEMU processes as described below.

Memory sharing between QEMU instances: Since each VM's device emulation is performed by its own QEMU process these processes need to share memory to communicate: DMA requests passed through VirtIO queues must be shared so that send and receives may be matched and each QEMU instance needs to be able to access any VM's memory to perform the copies between send and receive buffers. Moreover, a shared memory buffer must be allocated and mapped as the onboard memory of a PCI device to expose it to guests.

Our QEMU instances are slightly modified so that they allocate all of this memory from a shared memory pool. This is currently achieved by allocating this memory pool in one process with mmap and the MAP SHARED flag before creating the QEMU instances for the VMs. These instances are then created by forking this initial process.

One limitation of this implementation is that it cannot support a varying number of communicating VMs. However we plan to support this by allocating each QEMU instance's memory separately using a file backed mmap. QEMU instances would then be able to access each other's memories by mapping these files into their respective address spaces. Another possibility would be to run VM instances inside threads of a single process but this would require to make QEMU thread-safe.

DMA transfers: Whenever a guest signals that it has queued DMA requests through the VirtIO interface, the corresponding QEMU instance will dequeue and process these requests. Receives are stored in a per port queue which is shared among QEMU instances and for each send, the corresponding receive queue is searched for a matching receive. If one is found, a copy is performed between the send and receive buffers and the completion registers are updated.

Evaluation

To evaluate our virtual device implementation, we have developed a minimal MPI library which provides MPI Irecv, MPI Isend and MPI Wait communication primitives. It should be noted that most other MPI calls can be implemented on top of these basic functions.

Small messages (≤ 32 KB) are transmitted over the shared buffers provided by the virtual device. This 32KB limit has been determined empirically. Each MPI task receives messages in the shared memory block of the virtual port it has been attributed. This memory block is used as a producer/consumer circular buffer and is protected by a lock to prevent concurrent writes.

For larger messages, we use a rendez-vous protocol. The sender sends the message header to the shared buffer of the receiver. In turn, when the receiver finds a matching receive, it posts a DMA receive request to the virtual device. It then sends an acknowledgment to the sender which can post a DMA send request.

We evaluate the performance of our implementation on a pingpong benchmark. We measure the latency and bandwidth achieved between two host processes communicating using MPICH2 and between two processes on two VMs using our virtual device and minimal MPI implementation (VMPI). The machine used for this test has two quad-core Xeon E5345 (2.33Ghz) processors and 4GB of RAM. Processes or VMs are bound to different processors. Results are shown in Figure 5. Nemesis uses lockless data structures and minimizes the number of instructions on the critical path which explains its better performance in small message latency compared to our naive MPI implementation. By implementing Nemesis' communication algorithm in our MPI library we should be able to attain similar performance. The bandwidth graph shows that as long as we use the virtual device's shared buffers for communication (up to 32 KB), there are little difference between the native and virtualized case. For larger messages, the ability to perform direct copies between send and receive buffers allows us to outperform Nemesis even when messages don't fit in the cache. This result outlines an interesting property of virtualization: it can be used to implement optimizations that cannot be performed natively without introducing privileged code.

In this paper, we presented the design and implementation of a virtual device for efficient message passing between VMs which share the same host. Our evaluation shows that it achieves near native performance in MPI pingpong tests and can outperform native userspace MPI implementations without introducing privileged code on the host.

In the future, we intend to integrate support for our device as a channel in an existing MPI implementation so as to provide the whole MPI interface. This will allow a more thorough evaluation of our solution on real HPC applications. We also plan to extend our virtual device so that it supports additional features such as live migration. This will require a callback mechanism to allow the MPI library to suspend and resume communications appropriately when tasks are migrated. On an other note, we plan to experiment with specialized VM scheduling policies that take communications and NUMA effects into account to reduce overheads when there are more VMs than available cores.

Fig. 3 .

 3 Fig. 3. Architecture of the KVM hypervisor

Fig. 5 .

 5 Fig. 5. Results