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

[Markus Armbruster]

Luiz Capitulino <lcapitulino at redhat.com> writes:

> On Fri, 23 May 2014 00:50:38 -0300
> Marcelo Tosatti <mtosatti at 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.