KVM: selftests: Use vcpu_clear_cpuid_feature() to clear x2APIC

Add X86_FEATURE_X2APIC and use vcpu_clear_cpuid_feature() to clear x2APIC
support in the xAPIC state test.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-30-seanjc@google.com

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

@@ -83,6 +83,7 @@ struct kvm_x86_cpu_feature {
 #define	X86_FEATURE_SMX			KVM_X86_CPU_FEATURE(0x1, 0, ECX, 6)
 #define	X86_FEATURE_PDCM		KVM_X86_CPU_FEATURE(0x1, 0, ECX, 15)
 #define	X86_FEATURE_PCID		KVM_X86_CPU_FEATURE(0x1, 0, ECX, 17)
+#define X86_FEATURE_X2APIC		KVM_X86_CPU_FEATURE(0x1, 0, ECX, 21)
 #define	X86_FEATURE_MOVBE		KVM_X86_CPU_FEATURE(0x1, 0, ECX, 22)
 #define	X86_FEATURE_TSC_DEADLINE_TIMER	KVM_X86_CPU_FEATURE(0x1, 0, ECX, 24)
 #define	X86_FEATURE_XSAVE		KVM_X86_CPU_FEATURE(0x1, 0, ECX, 26)

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

@@ -136,9 +136,7 @@ int main(int argc, char *argv[])
 		.vcpu = NULL,
 		.is_x2apic = true,
 	};
-	struct kvm_cpuid2 *cpuid;
 	struct kvm_vm *vm;
-	int i;
 
 	vm = vm_create_with_one_vcpu(&x.vcpu, x2apic_guest_code);
 	test_icr(&x);
@@ -152,13 +150,7 @@ int main(int argc, char *argv[])
 	vm = vm_create_with_one_vcpu(&x.vcpu, xapic_guest_code);
 	x.is_x2apic = false;
 
-	cpuid = x.vcpu->cpuid;
-	for (i = 0; i < cpuid->nent; i++) {
-		if (cpuid->entries[i].function == 1)
-			break;
-	}
-	cpuid->entries[i].ecx &= ~BIT(21);
-	vcpu_set_cpuid(x.vcpu);
+	vcpu_clear_cpuid_feature(x.vcpu, X86_FEATURE_X2APIC);
 
 	virt_pg_map(vm, APIC_DEFAULT_GPA, APIC_DEFAULT_GPA);
 	test_icr(&x);