Re: [Qemu-devel] [libvirt] [PATCHv2 3/4] qemu: fix RTC_CHANGE event for <clock offset='variable' basis='utc'/>

["Daniel P. Berrange" <berrange@redhat.com>]

On Fri, May 23, 2014 at 10:48:18AM -0300, Marcelo Tosatti wrote:
> On Fri, May 23, 2014 at 03:35:19PM +0200, Markus Armbruster wrote:
> > Luiz Capitulino <lcapitulino@redhat.com> writes:
> > 
> > > On Fri, 23 May 2014 00:50:38 -0300
> > > Marcelo Tosatti <mtosatti@redhat.com> wrote:
> > >
> > >> > Then the guest triggers an RTC update, so qemu sends an event, but the
> > >> > event is lost. Then libvirtd starts again, and doesn't realize the
> > >> > event is lost.
> > >> 
> > >> Yes, but that case is also true for any other QMP asynchronous event,
> > >> and therefore should be handled generically i suppose (QMP channel data
> > >> should be maintained across libvirtd shutdown). Luiz?
> > >
> > > Maintaining QMP channel data doesn't solve this problem, because all sorts
> > > of race conditions are still possible. For example, libvirt could crash
> > > after having received the event but before handling it.
> > >
> > > The most reliable way we found to solve this problem, and that's what we
> > > do for other events, is to allow libvirt to query the information the event
> > > is reporting. An event is nothing more than a state change in QEMU, and QEMU
> > > state is persistent during the life time of the VM, so we allow libvirt to
> > > query the state of anything that may send an event.
> > 
> > In fact, this is a general rule: when libvirt tracks an event, it also
> > needs a way to poll for the information in the event.
> 
> I see.
> 
> This also seems pretty harmful wrt losing events:
> 
> /* Global, one-time initializer to configure the rate limiting
>  * and initialize state */
> static void monitor_protocol_event_init(void)
> {
>     /* Limit RTC & BALLOON events to 1 per second */
>     monitor_protocol_event_throttle(QEVENT_RTC_CHANGE, 1000);
> 
> Better remove it.

That is intentionally designed such that it doesn't cause any real
problems actually - the monitor rate limiting code will only drop
intermediate events - it is guaranteed you'll get the most recent
event after the rate limiting period elapse. eg if the guest OS
emits 6 events in the space on 1 second:

   RTC_CHANGE 353
   RTC_CHANGE 1338
   RTC_CHANGE 3542
   RTC_CHANGE 255
   RTC_CHANGE 522
   RTC_CHANGE 320

then, the monitor rate limiting may discard all except the very
last event. ie libvirt will see RTC_CHANGE == 320. The fact that
it didn't see the previous events is no problem, because they're
obsoleted by the new event.

Regards,
Daniel
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