KVM: nVMX: Prioritize SMI over nested IRQ/NMI

Check for an unblocked SMI in vmx_check_nested_events() so that pending
SMIs are correctly prioritized over IRQs and NMIs when the latter events
will trigger VM-Exit.  This also fixes an issue where an SMI that was
marked pending while processing a nested VM-Enter wouldn't trigger an
immediate exit, i.e. would be incorrectly delayed until L2 happened to
take a VM-Exit.

Fixes: 64d60670 ("KVM: x86: stubs for SMM support")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200423022550.15113-10-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -3750,6 +3750,12 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
 		return 0;
 	}
 
+	if (vcpu->arch.smi_pending && !is_smm(vcpu)) {
+		if (block_nested_events)
+			return -EBUSY;
+		goto no_vmexit;
+	}
+
 	if (vcpu->arch.nmi_pending && !vmx_nmi_blocked(vcpu)) {
 		if (block_nested_events)
 			return -EBUSY;