KVM: VMX: blocked-by-sti must not defer NMI injections

As the processor may not consider GUEST_INTR_STATE_STI as a reason for
blocking NMI, it could return immediately with EXIT_REASON_NMI_WINDOW
when we asked for it. But as we consider this state as NMI-blocking, we
can run into an endless loop.

Resolve this by allowing NMI injection if just GUEST_INTR_STATE_STI is
active (originally suggested by Gleb). Intel confirmed that this is
safe, the processor will never complain about NMI injection in this
state.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
KVM-Stable-Tag
Acked-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/vmx.c

@@ -2703,8 +2703,7 @@ static int vmx_nmi_allowed(struct kvm_vcpu *vcpu)
 		return 0;
 
 	return	!(vmcs_read32(GUEST_INTERRUPTIBILITY_INFO) &
-			(GUEST_INTR_STATE_STI | GUEST_INTR_STATE_MOV_SS |
-				GUEST_INTR_STATE_NMI));
+			(GUEST_INTR_STATE_MOV_SS | GUEST_INTR_STATE_NMI));
 }
 
 static bool vmx_get_nmi_mask(struct kvm_vcpu *vcpu)