Re: [Qemu-devel] Rethinking missed tick catchup

[Anthony Liguori <anthony@codemonkey.ws>]

Gleb Natapov <gleb@redhat.com> writes:

> On Thu, Sep 13, 2012 at 09:06:29AM -0500, Anthony Liguori wrote:
>> "Daniel P. Berrange" <berrange@redhat.com> writes:
>> 
>> I think it's better for QEMU to talk to qemu-ga.  We can tell when a large
>> period of time has passed in QEMU because we'll accumulate a large
>> number of missed ticks.
>> 
> With RTC configured to use vm clock we will not.

Not for host suspend.  For stop and live migration, we stop vm_clock.
But QEMU isn't aware of host suspend so vm_clock cannot be stopped.

>> This could happen because of stop, host suspend, live migration to a
>> file, etc.
>> 
>> It's much easier for us to call into qemu-ga to do the time correction
>> whenever this event occurs than to try and have libvirt figure out when
>> it's necessary.
> And if guest does not have qemu-ga what is better inject interrupts like
> crazy for next 2 minutes or leave guest with incorrect time?

Yes, at least that's fixable by the end-user.  QEMU consuming 100% CPU
for a prolonged period of time isn't fixable.

Regards,

Anthony Liguori


>
>> 
>> We know exactly when it's necessary, libvirt would need to guess.
>> 
>> Yes, we could generate a QMP event when a large skew was dedicated, but
>> I think this could happen often enough that it would be problematic.
>> Since QEMU is already implementing policy doing timer catchup in the
>> first place, I think we probably should own time catchup policy entirely.
>> 
>> Regards,
>> 
>> Anthony Liguori
>> 
>> >
>> > Regards,
>> > Daniel
>> > -- 
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>
> --
> 			Gleb.