Re: [Qemu-devel] [patch 0/2] add QMP command to reset rtc interrupt backlog

[Marcelo Tosatti]

On Mon, Jun 02, 2014 at 11:13:15AM +0200, Michal Privoznik wrote:
> On 30.05.2014 16:05, Marcelo Tosatti wrote:
> >It is necessary to reset RTC interrupt backlog if guest time is
> >synchronized via a different mechanism, such as QGA's guest-set-time
> >command.
> >
> >Failing to do so causes both corrections to be applied (summed),
> >resulting in an incorrect guest time.
> >
> >
> 
> Marcelo,
> 
> what happens if quest is synchronized via NTP? Does this command
> still need to be executed after 'cont'?
> 
> Michal

Yes because NTP will either take time to adjust by slewing or step the
clock (which renders guest-set-time() unuseable). During that time,
guest clock will be ahead of realtime.

Algorithm should be:

        -  ret = guest-set-time()
        -  if (ret = success)
        -       rtc-reset-reinjection()

That leaves a window where RTC interrupts are reinjected, but i 
see no better option.