[PATCH kernel 9/9] KVM: SVM: Don't try to pointlessly single-step SEV-ES guests for NMI window

[Alexey Kardashevskiy]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

Bail early from svm_enable_nmi_window() for SEV-ES guests without trying
to enable single-step of the guest, as single-stepping an SEV-ES guest is
impossible and the guest is responsible for *telling* KVM when it is ready
for an new NMI to be injected.

Functionally, setting TF and RF in svm->vmcb->save.rflags is benign as the
field is ignored by hardware, but it's all kinds of confusing.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
[aik: removed the clause about "KVM suppresses EFER.SVME (see efer_trap())"]
Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
---
Changes:
v6:
* new to the series
---
 arch/x86/kvm/svm/svm.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 52f1d88e82a0..c9837a8667b7 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3824,6 +3824,19 @@ static void svm_enable_nmi_window(struct kvm_vcpu *vcpu)
 	if (svm_get_nmi_mask(vcpu) && !svm->awaiting_iret_completion)
 		return; /* IRET will cause a vm exit */
 
+	/*
+	 * SEV-ES guests are responsible for signaling when a vCPU is ready to
+	 * receive a new NMI, as SEV-ES guests can't be single-stepped, i.e.
+	 * KVM can't intercept and single-step IRET to detect when NMIs are
+	 * unblocked (architecturally speaking).  See SVM_VMGEXIT_NMI_COMPLETE.
+	 *
+	 * Note, GIF is guaranteed to be '1' for SEV-ES guests as hardware
+	 * ignores SEV-ES guest writes to EFER.SVME *and* CLGI/STGI are not
+	 * supported NAEs in the GHCB protocol.
+	 */
+	if (sev_es_guest(vcpu->kvm))
+		return;
+
 	if (!gif_set(svm)) {
 		if (vgif)
 			svm_set_intercept(svm, INTERCEPT_STGI);
-- 
2.40.1