KVM: selftests: Fix vmxon_pa == vmcs12_pa == -1ull nVMX testcase for !eVMCS

The "vmxon_pa == vmcs12_pa == -1ull" test happens to work by accident: as
Enlightened VMCS is always supported, set_default_vmx_state() adds
'KVM_STATE_NESTED_EVMCS' to 'flags' and the following branch of
vmx_set_nested_state() is executed:

        if ((kvm_state->flags & KVM_STATE_NESTED_EVMCS) &&
            (!guest_can_use(vcpu, X86_FEATURE_VMX) ||
             !vmx->nested.enlightened_vmcs_enabled))
                        return -EINVAL;

as 'enlightened_vmcs_enabled' is false. In fact, "vmxon_pa == vmcs12_pa ==
-1ull" is a valid state when not tainted by wrong flags so the test should
aim for this branch:

        if (kvm_state->hdr.vmx.vmxon_pa == INVALID_GPA)
                return 0;

Test all this properly:
- Without KVM_STATE_NESTED_EVMCS in the flags, the expected return value is
'0'.
- With KVM_STATE_NESTED_EVMCS flag (when supported) set, the expected
return value is '-EINVAL' prior to enabling eVMCS and '0' after.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Tested-by: Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20231205103630.1391318-11-vkuznets@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: vittyvk

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

@@ -125,21 +125,25 @@ void test_vmx_nested_state(struct kvm_vcpu *vcpu)
 
 	/*
 	 * Setting vmxon_pa == -1ull and vmcs_pa == -1ull exits early without
-	 * setting the nested state but flags other than eVMCS must be clear.
-	 * The eVMCS flag can be set if the enlightened VMCS capability has
-	 * been enabled.
+	 * setting the nested state. When the eVMCS flag is not set, the
+	 * expected return value is '0'.
 	 */
 	set_default_vmx_state(state, state_sz);
+	state->flags = 0;
 	state->hdr.vmx.vmxon_pa = -1ull;
 	state->hdr.vmx.vmcs12_pa = -1ull;
-	test_nested_state_expect_einval(vcpu, state);
+	test_nested_state(vcpu, state);
 
-	state->flags &= KVM_STATE_NESTED_EVMCS;
+	/*
+	 * When eVMCS is supported, the eVMCS flag can only be set if the
+	 * enlightened VMCS capability has been enabled.
+	 */
 	if (have_evmcs) {
+		state->flags = KVM_STATE_NESTED_EVMCS;
 		test_nested_state_expect_einval(vcpu, state);
 		vcpu_enable_evmcs(vcpu);
+		test_nested_state(vcpu, state);
 	}
-	test_nested_state(vcpu, state);
 
 	/* It is invalid to have vmxon_pa == -1ull and SMM flags non-zero. */
 	state->hdr.vmx.smm.flags = 1;