KVM: nSVM: ignore L1 interrupt window while running L2 with V_INTR_MASKING=1

If a nested VM is started while an IRQ was pending and with
V_INTR_MASKING=1, the behavior of the guest depends on host IF.  If it
is 1, the VM should exit immediately, before executing the first
instruction of the guest, because VMRUN sets GIF back to 1.

If it is 0 and the host has VGIF, however, at the time of the VMRUN
instruction L0 is running the guest with a pending interrupt window
request.  This interrupt window request is completely irrelevant to
L2, since IF only controls virtual interrupts, so this patch drops
INTERCEPT_VINTR from the VMCB while running L2 under these circumstances.
To simplify the code, both steps of enabling the interrupt window
(setting the VINTR intercept and requesting a fake virtual interrupt
in svm_inject_irq) are grouped in the svm_set_vintr function, and
likewise for dismissing the interrupt window request in svm_clear_vintr.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm.c

@@ -528,6 +528,13 @@ static void recalc_intercepts(struct vcpu_svm *svm)
 		/* We only want the cr8 intercept bits of L1 */
 		c->intercept_cr &= ~(1U << INTERCEPT_CR8_READ);
 		c->intercept_cr &= ~(1U << INTERCEPT_CR8_WRITE);
+
+		/*
+		 * Once running L2 with HF_VINTR_MASK, EFLAGS.IF does not
+		 * affect any interrupt we may want to inject; therefore,
+		 * interrupt window vmexits are irrelevant to L0.
+		 */
+		c->intercept &= ~(1ULL << INTERCEPT_VINTR);
 	}
 
 	/* We don't want to see VMMCALLs from a nested guest */
@@ -641,6 +648,11 @@ static inline void clr_intercept(struct vcpu_svm *svm, int bit)
 	recalc_intercepts(svm);
 }
 
+static inline bool is_intercept(struct vcpu_svm *svm, int bit)
+{
+	return (svm->vmcb->control.intercept & (1ULL << bit)) != 0;
+}
+
 static inline bool vgif_enabled(struct vcpu_svm *svm)
 {
 	return !!(svm->vmcb->control.int_ctl & V_GIF_ENABLE_MASK);
@@ -2440,14 +2452,38 @@ static void svm_cache_reg(struct kvm_vcpu *vcpu, enum kvm_reg reg)
 	}
 }
 
+static inline void svm_enable_vintr(struct vcpu_svm *svm)
+{
+	struct vmcb_control_area *control;
+
+	/* The following fields are ignored when AVIC is enabled */
+	WARN_ON(kvm_vcpu_apicv_active(&svm->vcpu));
+
+	/*
+	 * This is just a dummy VINTR to actually cause a vmexit to happen.
+	 * Actual injection of virtual interrupts happens through EVENTINJ.
+	 */
+	control = &svm->vmcb->control;
+	control->int_vector = 0x0;
+	control->int_ctl &= ~V_INTR_PRIO_MASK;
+	control->int_ctl |= V_IRQ_MASK |
+		((/*control->int_vector >> 4*/ 0xf) << V_INTR_PRIO_SHIFT);
+	mark_dirty(svm->vmcb, VMCB_INTR);
+}
+
 static void svm_set_vintr(struct vcpu_svm *svm)
 {
 	set_intercept(svm, INTERCEPT_VINTR);
+	if (is_intercept(svm, INTERCEPT_VINTR))
+		svm_enable_vintr(svm);
 }
 
 static void svm_clear_vintr(struct vcpu_svm *svm)
 {
 	clr_intercept(svm, INTERCEPT_VINTR);
+
+	svm->vmcb->control.int_ctl &= ~V_IRQ_MASK;
+	mark_dirty(svm->vmcb, VMCB_INTR);
 }
 
 static struct vmcb_seg *svm_seg(struct kvm_vcpu *vcpu, int seg)
@@ -3835,11 +3871,8 @@ static int clgi_interception(struct vcpu_svm *svm)
 	disable_gif(svm);
 
 	/* After a CLGI no interrupts should come */
-	if (!kvm_vcpu_apicv_active(&svm->vcpu)) {
+	if (!kvm_vcpu_apicv_active(&svm->vcpu))
 		svm_clear_vintr(svm);
-		svm->vmcb->control.int_ctl &= ~V_IRQ_MASK;
-		mark_dirty(svm->vmcb, VMCB_INTR);
-	}
 
 	return ret;
 }
@@ -5125,19 +5158,6 @@ static void svm_inject_nmi(struct kvm_vcpu *vcpu)
 	++vcpu->stat.nmi_injections;
 }
 
-static inline void svm_inject_irq(struct vcpu_svm *svm, int irq)
-{
-	struct vmcb_control_area *control;
-
-	/* The following fields are ignored when AVIC is enabled */
-	control = &svm->vmcb->control;
-	control->int_vector = irq;
-	control->int_ctl &= ~V_INTR_PRIO_MASK;
-	control->int_ctl |= V_IRQ_MASK |
-		((/*control->int_vector >> 4*/ 0xf) << V_INTR_PRIO_SHIFT);
-	mark_dirty(svm->vmcb, VMCB_INTR);
-}
-
 static void svm_set_irq(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
@@ -5561,7 +5581,6 @@ static void enable_irq_window(struct kvm_vcpu *vcpu)
 		 */
 		svm_toggle_avic_for_irq_window(vcpu, false);
 		svm_set_vintr(svm);
-		svm_inject_irq(svm, 0x0);
 	}
 }