* Jeremy Fitzhardinge <jeremy@xxxxxxxx> [2012-01-18 12:34:42]:
> >> What prevents a kick from being lost here, if say, the waiter is at
> >> local_irq_save in kvm_lock_spinning, before the lock/want assignments?
> > The waiter does check for lock becoming available before actually
> > sleeping:
> >
> > + /*
> > + * check again make sure it didn't become free while
> > + * we weren't looking.
> > + */
> > + if (ACCESS_ONCE(lock->tickets.head) == want) {
> > + add_stats(TAKEN_SLOW_PICKUP, 1);
> > + goto out;
> > + }
>
> That logic relies on the "kick" being level triggered, so that "kick"
> before "block" will cause the block to fall out immediately. If you're
> using "hlt" as the block and it has the usual edge-triggered behaviour,
> what stops a "kick-before-hlt" from losing the kick?
Hmm ..'hlt' should result in a check for kick request (in hypervisor
context) before vcpu is put to sleep. IOW vcpu1 that is attempting to kick vcpu0
will set a 'somebody_tried_kicking_vcpu0' flag, which hypervisor should check
before it puts vcpu0 to sleep because of trapped 'hlt' instruction.
Won't that trap the 'kick-before-hlt' case? What am I missing here?
- vatsa
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