KVM: selftests: Use vcpu_get_cpuid_entry() in PV features test (sort of)

Add a new helper, vcpu_clear_cpuid_entry(), to do a RMW operation on the
vCPU's CPUID model to clear a given CPUID entry, and use it to clear
KVM's paravirt feature instead of operating on kvm_get_supported_cpuid()'s
static "cpuid" variable.  This also eliminates a user of
the soon-be-defunct set_cpuid() helper.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-26-seanjc@google.com

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

@@ -660,6 +660,7 @@ static inline void vcpu_set_cpuid(struct kvm_vcpu *vcpu)
 
 void vcpu_set_cpuid_maxphyaddr(struct kvm_vcpu *vcpu, uint8_t maxphyaddr);
 
+void vcpu_clear_cpuid_entry(struct kvm_vcpu *vcpu, uint32_t function);
 void vcpu_set_or_clear_cpuid_feature(struct kvm_vcpu *vcpu,
 				     struct kvm_x86_cpu_feature feature,
 				     bool set);

tools/testing/selftests/kvm/lib/x86_64/processor.c

@@ -770,6 +770,17 @@ void vcpu_set_cpuid_maxphyaddr(struct kvm_vcpu *vcpu, uint8_t maxphyaddr)
 	vcpu_set_cpuid(vcpu);
 }
 
+void vcpu_clear_cpuid_entry(struct kvm_vcpu *vcpu, uint32_t function)
+{
+	struct kvm_cpuid_entry2 *entry = vcpu_get_cpuid_entry(vcpu, function);
+
+	entry->eax = 0;
+	entry->ebx = 0;
+	entry->ecx = 0;
+	entry->edx = 0;
+	vcpu_set_cpuid(vcpu);
+}
+
 void vcpu_set_or_clear_cpuid_feature(struct kvm_vcpu *vcpu,
 				     struct kvm_x86_cpu_feature feature,
 				     bool set)

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

@@ -95,15 +95,6 @@ static void guest_main(void)
 	GUEST_DONE();
 }
 
-static void clear_kvm_cpuid_features(struct kvm_cpuid2 *cpuid)
-{
-	struct kvm_cpuid_entry2 ent = {0};
-
-	ent.function = KVM_CPUID_FEATURES;
-	TEST_ASSERT(set_cpuid(cpuid, &ent),
-		    "failed to clear KVM_CPUID_FEATURES leaf");
-}
-
 static void pr_msr(struct ucall *uc)
 {
 	struct msr_data *msr = (struct msr_data *)uc->args[0];
@@ -156,8 +147,7 @@ int main(void)
 
 	vcpu_enable_cap(vcpu, KVM_CAP_ENFORCE_PV_FEATURE_CPUID, 1);
 
-	clear_kvm_cpuid_features(vcpu->cpuid);
-	vcpu_set_cpuid(vcpu);
+	vcpu_clear_cpuid_entry(vcpu, KVM_CPUID_FEATURES);
 
 	vm_init_descriptor_tables(vm);
 	vcpu_init_descriptor_tables(vcpu);