Verify that KVM reports the actual number of CPUID entries on success, but
doesn't touch the userspace struct on failure (which for better or worse,
is KVM's ABI).

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 .../testing/selftests/kvm/x86_64/cpuid_test.c | 21 +++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/tools/testing/selftests/kvm/x86_64/cpuid_test.c b/tools/testing/selftests/kvm/x86_64/cpuid_test.c
index 2fc3ad9c887e..d3c3aa93f090 100644
--- a/tools/testing/selftests/kvm/x86_64/cpuid_test.c
+++ b/tools/testing/selftests/kvm/x86_64/cpuid_test.c
@@ -163,6 +163,25 @@ static void set_cpuid_after_run(struct kvm_vcpu *vcpu)
 	ent->eax = eax;
 }
 
+static void test_get_cpuid2(struct kvm_vcpu *vcpu)
+{
+	struct kvm_cpuid2 *cpuid = allocate_kvm_cpuid2(vcpu->cpuid->nent + 1);
+	int i, r;
+
+	vcpu_ioctl(vcpu, KVM_GET_CPUID2, cpuid);
+	TEST_ASSERT(cpuid->nent == vcpu->cpuid->nent,
+		    "KVM didn't update nent on success, wanted %u, got %u\n",
+		    vcpu->cpuid->nent, cpuid->nent);
+
+	for (i = 0; i < vcpu->cpuid->nent; i++) {
+		cpuid->nent = i;
+		r = __vcpu_ioctl(vcpu, KVM_GET_CPUID2, cpuid);
+		TEST_ASSERT(r && errno == E2BIG, KVM_IOCTL_ERROR(KVM_GET_CPUID2, r));
+		TEST_ASSERT(cpuid->nent == i, "KVM modified nent on failure");
+	}
+	free(cpuid);
+}
+
 int main(void)
 {
 	struct kvm_vcpu *vcpu;
@@ -183,5 +202,7 @@ int main(void)
 
 	set_cpuid_after_run(vcpu);
 
+	test_get_cpuid2(vcpu);
+
 	kvm_vm_free(vm);
 }
-- 
2.41.0.rc0.172.g3f132b7071-goog