[PATCH 10/12] evtchn/fifo: use stable fields when recording "last queue" information

[Jan Beulich <jbeulich@xxxxxxxx>]

Both evtchn->priority and evtchn->notify_vcpu_id could, prior to recent
locking adjustments, change behind the back of
evtchn_fifo_set_pending(). Neither the queue's priority nor the vCPU's
vcpu_id fields have similar properties, so they seem better suited for
the purpose. In particular they reflect the respective evtchn fields'
values at the time they were used to determine queue and vCPU.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -246,8 +246,8 @@ static void evtchn_fifo_set_pending(stru
         /* Moved to a different queue? */
         if ( old_q != q )
         {
-            evtchn->last_vcpu_id = evtchn->notify_vcpu_id;
-            evtchn->last_priority = evtchn->priority;
+            evtchn->last_vcpu_id = v->vcpu_id;
+            evtchn->last_priority = q->priority;
 
             spin_unlock_irqrestore(&old_q->lock, flags);
             spin_lock_irqsave(&q->lock, flags);