KVM: VMX: Avoid vmx_recover_nmi_blocking() when unneeded

When we haven't injected an interrupt, we don't need to recover
the nmi blocking state (since the guest can't set it by itself).
This allows us to avoid a VMREAD later on.
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/vmx.c

@@ -129,6 +129,7 @@ struct vcpu_vmx {
 	int                   launched;
 	u8                    fail;
 	u8                    cpl;
+	bool                  nmi_known_unmasked;
 	u32                   exit_intr_info;
 	u32                   idt_vectoring_info;
 	ulong                 rflags;
@@ -2959,6 +2960,7 @@ static void vmx_inject_nmi(struct kvm_vcpu *vcpu)
 	}
 
 	++vcpu->stat.nmi_injections;
+	vmx->nmi_known_unmasked = false;
 	if (vmx->rmode.vm86_active) {
 		if (kvm_inject_realmode_interrupt(vcpu, NMI_VECTOR) != EMULATE_DONE)
 			kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
@@ -2983,6 +2985,8 @@ static bool vmx_get_nmi_mask(struct kvm_vcpu *vcpu)
 {
 	if (!cpu_has_virtual_nmis())
 		return to_vmx(vcpu)->soft_vnmi_blocked;
+	if (to_vmx(vcpu)->nmi_known_unmasked)
+		return false;
 	return vmcs_read32(GUEST_INTERRUPTIBILITY_INFO)	& GUEST_INTR_STATE_NMI;
 }
 
@@ -2996,6 +3000,7 @@ static void vmx_set_nmi_mask(struct kvm_vcpu *vcpu, bool masked)
 			vmx->vnmi_blocked_time = 0;
 		}
 	} else {
+		vmx->nmi_known_unmasked = !masked;
 		if (masked)
 			vmcs_set_bits(GUEST_INTERRUPTIBILITY_INFO,
 				      GUEST_INTR_STATE_NMI);
@@ -3527,9 +3532,11 @@ static int handle_task_switch(struct kvm_vcpu *vcpu)
 		switch (type) {
 		case INTR_TYPE_NMI_INTR:
 			vcpu->arch.nmi_injected = false;
-			if (cpu_has_virtual_nmis())
+			if (cpu_has_virtual_nmis()) {
 				vmcs_set_bits(GUEST_INTERRUPTIBILITY_INFO,
 					      GUEST_INTR_STATE_NMI);
+				vmx->nmi_known_unmasked = false;
+			}
 			break;
 		case INTR_TYPE_EXT_INTR:
 		case INTR_TYPE_SOFT_INTR:
@@ -3916,6 +3923,8 @@ static void vmx_recover_nmi_blocking(struct vcpu_vmx *vmx)
 	idtv_info_valid = vmx->idt_vectoring_info & VECTORING_INFO_VALID_MASK;
 
 	if (cpu_has_virtual_nmis()) {
+		if (vmx->nmi_known_unmasked)
+			return;
 		unblock_nmi = (exit_intr_info & INTR_INFO_UNBLOCK_NMI) != 0;
 		vector = exit_intr_info & INTR_INFO_VECTOR_MASK;
 		/*
@@ -3932,6 +3941,10 @@ static void vmx_recover_nmi_blocking(struct vcpu_vmx *vmx)
 		    vector != DF_VECTOR && !idtv_info_valid)
 			vmcs_set_bits(GUEST_INTERRUPTIBILITY_INFO,
 				      GUEST_INTR_STATE_NMI);
+		else
+			vmx->nmi_known_unmasked =
+				!(vmcs_read32(GUEST_INTERRUPTIBILITY_INFO)
+				  & GUEST_INTR_STATE_NMI);
 	} else if (unlikely(vmx->soft_vnmi_blocked))
 		vmx->vnmi_blocked_time +=
 			ktime_to_ns(ktime_sub(ktime_get(), vmx->entry_time));
@@ -3970,6 +3983,7 @@ static void __vmx_complete_interrupts(struct vcpu_vmx *vmx,
 		 */
 		vmcs_clear_bits(GUEST_INTERRUPTIBILITY_INFO,
 				GUEST_INTR_STATE_NMI);
+		vmx->nmi_known_unmasked = true;
 		break;
 	case INTR_TYPE_SOFT_EXCEPTION:
 		vmx->vcpu.arch.event_exit_inst_len =