KVM: SVM: detect opening of SMI window using STGI intercept

Commit 05cade7 ("KVM: nSVM: fix SMI injection in guest mode") made
KVM mask SMI if GIF=0 but it didn't do anything to unmask it when GIF is
enabled.

The issue manifests for me as a significantly longer boot time of Windows
guests when running with SMM-enabled OVMF.

This commit fixes it by intercepting STGI instead of requesting immediate
exit if the reason why SMM was masked is GIF.

Fixes: 05cade7 ("KVM: nSVM: fix SMI injection in guest mode")
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1065,6 +1065,7 @@ struct kvm_x86_ops {
 	int (*smi_allowed)(struct kvm_vcpu *vcpu);
 	int (*pre_enter_smm)(struct kvm_vcpu *vcpu, char *smstate);
 	int (*pre_leave_smm)(struct kvm_vcpu *vcpu, u64 smbase);
+	int (*enable_smi_window)(struct kvm_vcpu *vcpu);
 };
 
 struct kvm_arch_async_pf {

arch/x86/kvm/svm.c

@@ -3187,7 +3187,7 @@ static int stgi_interception(struct vcpu_svm *svm)
 
 	/*
 	 * If VGIF is enabled, the STGI intercept is only added to
-	 * detect the opening of the NMI window; remove it now.
+	 * detect the opening of the SMI/NMI window; remove it now.
 	 */
 	if (vgif_enabled(svm))
 		clr_intercept(svm, INTERCEPT_STGI);
@@ -5476,6 +5476,19 @@ static int svm_pre_leave_smm(struct kvm_vcpu *vcpu, u64 smbase)
 	return ret;
 }
 
+static int enable_smi_window(struct kvm_vcpu *vcpu)
+{
+	struct vcpu_svm *svm = to_svm(vcpu);
+
+	if (!gif_set(svm)) {
+		if (vgif_enabled(svm))
+			set_intercept(svm, INTERCEPT_STGI);
+		/* STGI will cause a vm exit */
+		return 1;
+	}
+	return 0;
+}
+
 static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
 	.cpu_has_kvm_support = has_svm,
 	.disabled_by_bios = is_disabled,
@@ -5590,6 +5603,7 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
 	.smi_allowed = svm_smi_allowed,
 	.pre_enter_smm = svm_pre_enter_smm,
 	.pre_leave_smm = svm_pre_leave_smm,
+	.enable_smi_window = enable_smi_window,
 };
 
 static int __init svm_init(void)

arch/x86/kvm/vmx.c

@@ -11973,6 +11973,11 @@ static int vmx_pre_leave_smm(struct kvm_vcpu *vcpu, u64 smbase)
 	return 0;
 }
 
+static int enable_smi_window(struct kvm_vcpu *vcpu)
+{
+	return 0;
+}
+
 static struct kvm_x86_ops vmx_x86_ops __ro_after_init = {
 	.cpu_has_kvm_support = cpu_has_kvm_support,
 	.disabled_by_bios = vmx_disabled_by_bios,
@@ -12102,6 +12107,7 @@ static struct kvm_x86_ops vmx_x86_ops __ro_after_init = {
 	.smi_allowed = vmx_smi_allowed,
 	.pre_enter_smm = vmx_pre_enter_smm,
 	.pre_leave_smm = vmx_pre_leave_smm,
+	.enable_smi_window = enable_smi_window,
 };
 
 static int __init vmx_init(void)

arch/x86/kvm/x86.c

@@ -6892,17 +6892,23 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		if (inject_pending_event(vcpu, req_int_win) != 0)
 			req_immediate_exit = true;
 		else {
-			/* Enable NMI/IRQ window open exits if needed.
+			/* Enable SMI/NMI/IRQ window open exits if needed.
 			 *
-			 * SMIs have two cases: 1) they can be nested, and
-			 * then there is nothing to do here because RSM will
-			 * cause a vmexit anyway; 2) or the SMI can be pending
-			 * because inject_pending_event has completed the
-			 * injection of an IRQ or NMI from the previous vmexit,
-			 * and then we request an immediate exit to inject the SMI.
+			 * SMIs have three cases:
+			 * 1) They can be nested, and then there is nothing to
+			 *    do here because RSM will cause a vmexit anyway.
+			 * 2) There is an ISA-specific reason why SMI cannot be
+			 *    injected, and the moment when this changes can be
+			 *    intercepted.
+			 * 3) Or the SMI can be pending because
+			 *    inject_pending_event has completed the injection
+			 *    of an IRQ or NMI from the previous vmexit, and
+			 *    then we request an immediate exit to inject the
+			 *    SMI.
 			 */
 			if (vcpu->arch.smi_pending && !is_smm(vcpu))
-				req_immediate_exit = true;
+				if (!kvm_x86_ops->enable_smi_window(vcpu))
+					req_immediate_exit = true;
 			if (vcpu->arch.nmi_pending)
 				kvm_x86_ops->enable_nmi_window(vcpu);
 			if (kvm_cpu_has_injectable_intr(vcpu) || req_int_win)