KVM: only abort guest entry if timer count goes from 0->1

Only abort guest entry if the timer count went from 0->1, since for 1->2
or larger the bit will either be set already or a timer irq will have
been injected.

Using atomic_inc_and_test() for it also introduces an SMP barrier
to the LAPIC version (thought it was unecessary because of timer
migration, but guest can be scheduled to a different pCPU between exit
and kvm_vcpu_block(), so there is the possibility for a race).

Noticed by Avi.

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

1 files changed, 4 insertions, 7 deletions

diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
index 9e3391e9a1b7..c0f7872a9124 100644
--- a/arch/x86/kvm/i8254.c
+++ b/arch/x86/kvm/i8254.c
@@ -198,14 +198,11 @@ static int __pit_timer_fn(struct kvm_kpit_state *ps)
         struct kvm_vcpu *vcpu0 = ps->pit->kvm->vcpus[0];
         struct kvm_kpit_timer *pt = &ps->pit_timer;
 
-        atomic_inc(&pt->pending);
-        smp_mb__after_atomic_inc();
-        if (vcpu0) {
+        if (!atomic_inc_and_test(&pt->pending))
                 set_bit(KVM_REQ_PENDING_TIMER, &vcpu0->requests);
-                if (waitqueue_active(&vcpu0->wq)) {
-                        vcpu0->arch.mp_state = KVM_MP_STATE_RUNNABLE;
-                        wake_up_interruptible(&vcpu0->wq);
-                }
+        if (vcpu0 && waitqueue_active(&vcpu0->wq)) {
+                vcpu0->arch.mp_state = KVM_MP_STATE_RUNNABLE;
+                wake_up_interruptible(&vcpu0->wq);
         }
 
         pt->timer.expires = ktime_add_ns(pt->timer.expires, pt->period);