KVM: Fix simultaneous NMIs

If simultaneous NMIs happen, we're supposed to queue the second
and next (collapsing them), but currently we sometimes collapse
the second into the first.

Fix by using a counter for pending NMIs instead of a bool; since
the counter limit depends on whether the processor is currently
in an NMI handler, which can only be checked in vcpu context
(via the NMI mask), we add a new KVM_REQ_NMI to request recalculation
of the counter.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -413,8 +413,9 @@ struct kvm_vcpu_arch {
 	u32  tsc_catchup_mult;
 	s8   tsc_catchup_shift;
 
-	bool nmi_pending;
-	bool nmi_injected;
+	atomic_t nmi_queued;  /* unprocessed asynchronous NMIs */
+	unsigned nmi_pending; /* NMI queued after currently running handler */
+	bool nmi_injected;    /* Trying to inject an NMI this entry */
 
 	struct mtrr_state_type mtrr_state;
 	u32 pat;