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>
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index e10217e..e53a5f7 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -4417,22 +4417,20 @@
 	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) {
-		enable_irq_window(vcpu);
-		return;
-	}
+	if (vmcs_read32(GUEST_INTERRUPTIBILITY_INFO) & GUEST_INTR_STATE_STI)
+		return enable_irq_window(vcpu);
+
 	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)