KVM: x86: Account for failing enable_irq_window for NMI window request

With VMX, enable_irq_window can now return -EBUSY, in which case an
immediate exit shall be requested before entering the guest. Account for
this also in enable_nmi_window which uses enable_irq_window in absence
of vnmi support, e.g.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -695,7 +695,7 @@ struct kvm_x86_ops {
 	int (*nmi_allowed)(struct kvm_vcpu *vcpu);
 	bool (*get_nmi_mask)(struct kvm_vcpu *vcpu);
 	void (*set_nmi_mask)(struct kvm_vcpu *vcpu, bool masked);
-	void (*enable_nmi_window)(struct kvm_vcpu *vcpu);
+	int (*enable_nmi_window)(struct kvm_vcpu *vcpu);
 	int (*enable_irq_window)(struct kvm_vcpu *vcpu);
 	void (*update_cr8_intercept)(struct kvm_vcpu *vcpu, int tpr, int irr);
 	int (*vm_has_apicv)(struct kvm *kvm);

arch/x86/kvm/svm.c

@@ -3649,13 +3649,13 @@ static int enable_irq_window(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
-static void enable_nmi_window(struct kvm_vcpu *vcpu)
+static int enable_nmi_window(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
 	if ((svm->vcpu.arch.hflags & (HF_NMI_MASK | HF_IRET_MASK))
 	    == HF_NMI_MASK)
-		return; /* IRET will cause a vm exit */
+		return 0; /* IRET will cause a vm exit */
 
 	/*
 	 * Something prevents NMI from been injected. Single step over possible
@@ -3664,6 +3664,7 @@ static void enable_nmi_window(struct kvm_vcpu *vcpu)
 	svm->nmi_singlestep = true;
 	svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
 	update_db_bp_intercept(vcpu);
+	return 0;
 }
 
 static int svm_set_tss_addr(struct kvm *kvm, unsigned int addr)

arch/x86/kvm/vmx.c

@@ -4417,22 +4417,20 @@ static int enable_irq_window(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
-static void enable_nmi_window(struct kvm_vcpu *vcpu)
+static int enable_nmi_window(struct kvm_vcpu *vcpu)
 {
 	u32 cpu_based_vm_exec_control;
 
-	if (!cpu_has_virtual_nmis()) {
-		enable_irq_window(vcpu);
-		return;
-	}
+	if (!cpu_has_virtual_nmis())
+		return enable_irq_window(vcpu);
+
+	if (vmcs_read32(GUEST_INTERRUPTIBILITY_INFO) & GUEST_INTR_STATE_STI)
+		return enable_irq_window(vcpu);
 
-	if (vmcs_read32(GUEST_INTERRUPTIBILITY_INFO) & GUEST_INTR_STATE_STI) {
-		enable_irq_window(vcpu);
-		return;
-	}
 	cpu_based_vm_exec_control = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
 	cpu_based_vm_exec_control |= CPU_BASED_VIRTUAL_NMI_PENDING;
 	vmcs_write32(CPU_BASED_VM_EXEC_CONTROL, cpu_based_vm_exec_control);
+	return 0;
 }
 
 static void vmx_inject_irq(struct kvm_vcpu *vcpu)

arch/x86/kvm/x86.c

@@ -5756,7 +5756,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 
 		/* enable NMI/IRQ window open exits if needed */
 		if (vcpu->arch.nmi_pending)
-			kvm_x86_ops->enable_nmi_window(vcpu);
+			req_immediate_exit =
+				kvm_x86_ops->enable_nmi_window(vcpu) != 0;
 		else if (kvm_cpu_has_injectable_intr(vcpu) || req_int_win)
 			req_immediate_exit =
 				kvm_x86_ops->enable_irq_window(vcpu) != 0;