KVM: SVM: Don't defer NMI unblocking until next exit for SEV-ES guests

Immediately mark NMIs as unmasked in response to #VMGEXIT(NMI complete)
instead of setting awaiting_iret_completion and waiting until the *next*
VM-Exit to unmask NMIs.  The whole point of "NMI complete" is that the
guest is responsible for telling the hypervisor when it's safe to inject
an NMI, i.e. there's no need to wait.  And because there's no IRET to
single-step, the next VM-Exit could be a long time coming, i.e. KVM could
incorrectly hold an NMI pending for far longer than what is required and
expected.

Opportunistically fix a stale reference to HF_IRET_MASK.

Fixes: 916b54a7 ("KVM: x86: Move HF_NMI_MASK and HF_IRET_MASK into "struct vcpu_svm"")
Fixes: 4444dfe4 ("KVM: SVM: Add NMI support for an SEV-ES guest")
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20230615063757.3039121-9-aik@amd.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/svm/sev.c

@@ -2900,7 +2900,10 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 					    svm->sev_es.ghcb_sa);
 		break;
 	case SVM_VMGEXIT_NMI_COMPLETE:
-		ret = svm_invoke_exit_handler(vcpu, SVM_EXIT_IRET);
+		++vcpu->stat.nmi_window_exits;
+		svm->nmi_masked = false;
+		kvm_make_request(KVM_REQ_EVENT, vcpu);
+		ret = 1;
 		break;
 	case SVM_VMGEXIT_AP_HLT_LOOP:
 		ret = kvm_emulate_ap_reset_hold(vcpu);

arch/x86/kvm/svm/svm.c

@@ -2535,12 +2535,13 @@ static int iret_interception(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
+	WARN_ON_ONCE(sev_es_guest(vcpu->kvm));
+
 	++vcpu->stat.nmi_window_exits;
 	svm->awaiting_iret_completion = true;
 
 	svm_clr_iret_intercept(svm);
-	if (!sev_es_guest(vcpu->kvm))
-		svm->nmi_iret_rip = kvm_rip_read(vcpu);
+	svm->nmi_iret_rip = kvm_rip_read(vcpu);
 
 	kvm_make_request(KVM_REQ_EVENT, vcpu);
 	return 1;
@@ -3950,12 +3951,11 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu)
 	svm->soft_int_injected = false;
 
 	/*
-	 * If we've made progress since setting HF_IRET_MASK, we've
+	 * If we've made progress since setting awaiting_iret_completion, we've
 	 * executed an IRET and can allow NMI injection.
 	 */
 	if (svm->awaiting_iret_completion &&
-	    (sev_es_guest(vcpu->kvm) ||
-	     kvm_rip_read(vcpu) != svm->nmi_iret_rip)) {
+	    kvm_rip_read(vcpu) != svm->nmi_iret_rip) {
 		svm->awaiting_iret_completion = false;
 		svm->nmi_masked = false;
 		kvm_make_request(KVM_REQ_EVENT, vcpu);