libstore: Add apple-virt to system features when available

I'm sure that we'll adjust the implementation over time, but this
at least discerns between an apple silicon bare metal machine and
a tart VM.

(cherry picked from commit 9277eb276bf0a942e88fcf499f6a6b9c262be853)

src/libstore/globals.cc

@@ -24,6 +24,9 @@
 
 #include "config-impl.hh"
 
+#ifdef __APPLE__
+#include <sys/sysctl.h>
+#endif
 
 namespace nix {
 
@@ -154,6 +157,29 @@ unsigned int Settings::getDefaultCores()
       return concurrency;
 }
 
+#if __APPLE__
+static bool hasVirt() {
+
+    int hasVMM;
+    int hvSupport;
+    size_t size;
+
+    size = sizeof(hasVMM);
+    if (sysctlbyname("kern.hv_vmm_present", &hasVMM, &size, NULL, 0) == 0) {
+        if (hasVMM)
+            return false;
+    }
+
+    // whether the kernel and hardware supports virt
+    size = sizeof(hvSupport);
+    if (sysctlbyname("kern.hv_support", &hvSupport, &size, NULL, 0) == 0) {
+        return hvSupport == 1;
+    } else {
+        return false;
+    }
+}
+#endif
+
 StringSet Settings::getDefaultSystemFeatures()
 {
     /* For backwards compatibility, accept some "features" that are
@@ -170,6 +196,11 @@ StringSet Settings::getDefaultSystemFeatures()
         features.insert("kvm");
     #endif
 
+    #if __APPLE__
+    if (hasVirt())
+        features.insert("apple-virt");
+    #endif
+
     return features;
 }
 

src/libstore/globals.hh

@@ -708,6 +708,7 @@ public:
           `kvm` feature.
 
           This setting by default includes `kvm` if `/dev/kvm` is accessible,
+          `apple-virt` if hardware virtualization is available on macOS,
           and the pseudo-features `nixos-test`, `benchmark` and `big-parallel`
           that are used in Nixpkgs to route builds to specific machines.
         )", {}, false};