KVM: SEV: Document the SEV-ES check when querying SMM support as "safe"

Use the "unsafe" API to check for an SEV-ES+ guest when determining whether
or not SMBASE is a supported MSR, i.e. whether or not emulated SMM is
supported.  This will eventually allow adding lockdep assertings to the
APIs for detecting SEV+ VMs without triggering "real" false positives.

While svm_has_emulated_msr() doesn't hold kvm->lock, i.e. can get both
false positives *and* false negatives, both are completely fine, as the
only time the result isn't stable is when userspace is the sole consumer
of the result.  I.e. userspace can confuse itself, but that's it.

No functional change intended.

Link: https://patch.msgid.link/20260310234829.2608037-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index d874af3d520a..69a3efc14368 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4517,9 +4517,17 @@ static bool svm_has_emulated_msr(struct kvm *kvm, u32 index)
 	case MSR_IA32_SMBASE:
 		if (!IS_ENABLED(CONFIG_KVM_SMM))
 			return false;
-		/* SEV-ES guests do not support SMM, so report false */
-		if (kvm && sev_es_guest(kvm))
+
+#ifdef CONFIG_KVM_AMD_SEV
+		/*
+		 * KVM can't access register state to emulate SMM for SEV-ES
+		 * guests.  Conusming stale data here is "fine", as KVM only
+		 * checks for MSR_IA32_SMBASE support without a vCPU when
+		 * userspace is querying KVM_CAP_X86_SMM.
+		 */
+		if (kvm && ____sev_es_guest(kvm))
 			return false;
+#endif
 		break;
 	default:
 		break;