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Set kern.curproc_arch_affinity=0 to escape Rosetta

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By default, once you enter x86_64 Rosetta 2, macOS will try to run
everything in x86_64. So an x86_64 Nix will still try to use x86_64
even when system = aarch64-darwin. To avoid this we can set
kern.curproc_arch_affinity sysctl. With kern.curproc_arch_affinity=0,
we ignore this preference.

This is based on how
https://opensource.apple.com/source/system_cmds/system_cmds-880.40.5/arch.tproj/arch.c.auto.html
works. Completely undocumented, but seems to work!

Note, you can verify this works with this impure Nix expression:

```
  {
    a = derivation {
      name = "a";
      system = "aarch64-darwin";
      builder = "/bin/sh";
      args = [ "-e" (builtins.toFile "builder" ''
        [ "$(/usr/bin/arch)" = arm64 ]
        [ "$(/usr/bin/arch -arch x86_64 /bin/sh -c /usr/bin/arch)" = i386 ]
        [ "$(/usr/bin/arch -arch arm64 /bin/sh -c /usr/bin/arch)" = arm64 ]
        /usr/bin/touch $out
      '') ];
    };

    b = derivation {
      name = "b";
      system = "x86_64-darwin";
      builder = "/bin/sh";
      args = [ "-e" (builtins.toFile "builder" ''
        [ "$(/usr/bin/arch)" = i386 ]
        [ "$(/usr/bin/arch -arch x86_64 /bin/sh -c /usr/bin/arch)" = i386 ]
        [ "$(/usr/bin/arch -arch arm64 /bin/sh -c /usr/bin/arch)" = arm64 ]
        /usr/bin/touch $out
      '') ];
    };
  }
```
This commit is contained in:
Matthew Bauer 2021-01-11 22:05:32 -06:00
parent 6254b1f5d2
commit f69820417f
1 changed files with 5 additions and 0 deletions
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5
src/libstore/build/derivation-goal.cc
View file

@ -52,6 +52,7 @@
#if __APPLE__
#include <spawn.h>
#include <sys/sysctl.h>
#endif
#include <pwd.h>
@ -2869,6 +2870,10 @@ void DerivationGoal::runChild()
throw SysError("failed to initialize builder");
if (drv->platform == "aarch64-darwin") {
// Unset kern.curproc_arch_affinity so we can escape Rosetta
int affinity = 0;
sysctlbyname("kern.curproc_arch_affinity", NULL, NULL, &affinity, sizeof(affinity));
cpu_type_t cpu = CPU_TYPE_ARM64;
posix_spawnattr_setbinpref_np(&attrp, 1, &cpu, NULL);
} else if (drv->platform == "x86_64-darwin") {