Re: [Qemu-devel] Rethinking missed tick catchup

[Avi Kivity <avi@redhat.com>]

On 09/13/2012 06:56 PM, Anthony Liguori wrote:
>>> 
>> Hmm, true. What about hooking into suspend and doing vmstop during
>> suspend. 
> 
> Is suspend the only foreseeable way for this problem to happen?  I don't
> think it is which is what concerns me about any approach that relies on
> "hooking suspend".

No, SIGSTOP/SIGCONT (can hook SIGCONT), gdb (can't hook but is very
rare), ENOSPACE + wait for more space to be provisioned (already known
to qemu), NFS access qemu core on dead server, severe swapstorms.

> Also, I don't think there is a generic way to "hook suspend".

That is what we have Lennart for.

>>> >> 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.
>>> 
>> You mean yes to "leave guest with incorrect time"? QEMU will still
>> consume 100% of cpu for some time calling qemu_timer callback millions
>> times. timedrift code is not the right level to fix that.
> 
> Not if we put a cap on how many interrupts we'll try to catch up.
> 
> As I mentioned previously, if we acrue more than X number of missed
> ticks, we should simply declare bankruptcy and reset the counter.

If we know we're missing N ticks, we can simply pass N to the handler.

> 
> When that occurs, *if* qemu-ga is present, we should ask qemu-ga to
> reset the guest's clock based on reading the hardware clock via a
> 'guest-resync-time' command.
> 
> If it isn't, time will be off.  Hopefully the guest is running NTP and
> can correct itself.  Otherwise, at least the admin can manually fix the
> time.

There is also the fake S3 (post host resume) that can get the guest to
read its RTC.


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