KVM: selftests: Add this_cpu_has() to query X86_FEATURE_* via cpuid()

Add this_cpu_has() to query an X86_FEATURE_* via cpuid(), i.e. to query a
feature from L1 (or L2) guest code.  Arbitrarily select the AMX test to
be the first user.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-33-seanjc@google.com

tools/testing/selftests/kvm/include/x86_64/processor.h

@@ -161,7 +161,6 @@ struct kvm_x86_cpu_feature {
 #define X86_FEATURE_KVM_MIGRATION_CONTROL	KVM_X86_CPU_FEATURE(0x40000001, 0, EAX, 17)
 
 /* CPUID.1.ECX */
-#define CPUID_XSAVE		(1ul << 26)
 #define CPUID_OSXSAVE		(1ul << 27)
 
 /* Page table bitfield declarations */
@@ -427,6 +426,17 @@ static inline void cpuid(uint32_t function,
 	return __cpuid(function, 0, eax, ebx, ecx, edx);
 }
 
+static inline bool this_cpu_has(struct kvm_x86_cpu_feature feature)
+{
+	uint32_t gprs[4];
+
+	__cpuid(feature.function, feature.index,
+		&gprs[KVM_CPUID_EAX], &gprs[KVM_CPUID_EBX],
+		&gprs[KVM_CPUID_ECX], &gprs[KVM_CPUID_EDX]);
+
+	return gprs[feature.reg] & BIT(feature.bit);
+}
+
 #define SET_XMM(__var, __xmm) \
 	asm volatile("movq %0, %%"#__xmm : : "r"(__var) : #__xmm)
 

tools/testing/selftests/kvm/x86_64/amx_test.c

@@ -120,13 +120,8 @@ static inline void __xsavec(struct xsave_data *data, uint64_t rfbm)
 
 static inline void check_cpuid_xsave(void)
 {
-	uint32_t eax, ebx, ecx, edx;
-
-	cpuid(1, &eax, &ebx, &ecx, &edx);
-	if (!(ecx & CPUID_XSAVE))
-		GUEST_ASSERT(!"cpuid: no CPU xsave support!");
-	if (!(ecx & CPUID_OSXSAVE))
-		GUEST_ASSERT(!"cpuid: no OS xsave support!");
+	GUEST_ASSERT(this_cpu_has(X86_FEATURE_XSAVE));
+	GUEST_ASSERT(this_cpu_has(X86_FEATURE_OSXSAVE));
 }
 
 static bool check_xsave_supports_xtile(void)