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

[Luiz Capitulino]

On Fri, 23 May 2014 00:50:38 -0300
Marcelo Tosatti <address@hidden> 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.