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.

doc/manual/src/release-notes/rl-next.md

@@ -8,6 +8,8 @@
 
 - Introduce new flake installable syntax `flakeref#.attrPath` where the "." prefix denotes no searching of default attribute prefixes like `packages.<SYSTEM>` or `legacyPackages.<SYSTEM>`.
 
+- Nix adds `apple-virt` to the default system features on macOS systems that support virtualization. This is similar to what's done for the `kvm` system feature on Linux hosts.
+
 - Introduce a new built-in function [`builtins.convertHash`](@docroot@/language/builtins.md#builtins-convertHash).
 
 - `builtins.fetchTree` is now marked as stable.

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

@@ -714,6 +714,10 @@ public:
 
           System features are user-defined, but Nix sets the following defaults:
 
+          - `apple-virt`
+
+            Included on darwin if virtualization is available.
+
           - `kvm`
 
             Included by default if `/dev/kvm` is accessible.