cpuid: cache host cpuid features

HostFeatureSet() returns a fixed set that is initialized at startup.

Here results fro the signal_benchmark on the kvm platform:
Before:
BM_FaultSignalFixup/real_time       8353 ns         8383 ns        94234

After:
BM_FaultSignalFixup/real_time       6127 ns         6087 ns       111708
PiperOrigin-RevId: 462012597

pkg/cpuid/native_amd64.go

@@ -162,13 +162,13 @@ func (fs FeatureSet) query(fn cpuidFunction) (uint32, uint32, uint32, uint32) {
 	return out.Eax, out.Ebx, out.Ecx, out.Edx
 }
 
+var hostFeatureSet FeatureSet
+
 // HostFeatureSet returns a host CPUID.
 //
 //go:nosplit
 func HostFeatureSet() FeatureSet {
-	return FeatureSet{
-		Function: &Native{},
-	}
+	return hostFeatureSet
 }
 
 var (
@@ -179,7 +179,7 @@ var (
 // Reads max cpu frequency from host /proc/cpuinfo. Must run before syscall
 // filter installation. This value is used to create the fake /proc/cpuinfo
 // from a FeatureSet.
-func init() {
+func readMaxCPUFreq() {
 	cpuinfob, err := ioutil.ReadFile("/proc/cpuinfo")
 	if err != nil {
 		// Leave it as 0... the VDSO bails out in the same way.
@@ -212,4 +212,13 @@ func init() {
 		}
 	}
 	log.Warningf("Could not parse /proc/cpuinfo, it is empty or does not contain cpu MHz")
+
+}
+
+func init() {
+	hostFeatureSet = FeatureSet{
+		Function: &Native{},
+	}.Fixed()
+
+	readMaxCPUFreq()
 }

pkg/sentry/platform/kvm/kvm_amd64_test.go

@@ -99,7 +99,9 @@ func nestedVirtIsOn(c *vCPU, fs *cpuid.FeatureSet) bool {
 
 func TestKernelCPUID(t *testing.T) {
 	bluepillTest(t, func(c *vCPU) {
-		fs := cpuid.HostFeatureSet()
+		fs := cpuid.FeatureSet{
+			Function: &cpuid.Native{},
+		}
 		if nestedVirtIsOn(c, &fs) {
 			t.Fatalf("Nested virtualization is enabled")
 		}