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

Luiz Capitulino <lcapitulino(a)redhat.com&gt; writes: > On Fri, 23 May 2014 00:50:38 -0300 > Marcelo Tosatti <mtosatti(a)redhat.com&gt; 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.