Re: [Qemu-devel] Rethinking missed tick catchup

[Daniel P. Berrange]

On Thu, Sep 13, 2012 at 07:14:08AM -0600, Eric Blake wrote:
> On 09/13/2012 04:49 AM, Gleb Natapov wrote:
> >> They do if you hibernate your laptop.
> >>
> > AFAIK libvirt migrates vm into a file on hibernate. It is better to move to 
> > S3
> > (using qemu-ga) instead and migrate to file only if s3 fails.
> 
> On host hibernate, libvirt currently does nothing to the guest.  When
> the host resumes, the guests see a large gap in execution.
> 
> Libvirt would need a hook into host hibernation, to have enough time to
> tell the guests to go into S3 prior to allowing the host to go into S3.
> 
> On host reboot, libvirt currently saves guests to disk using migrate to
> file.  The ideal solution would be to first tell the guest to go into S3
> before migrating to file, but the migration to file STILL must occur,
> because the host is about to reboot and S3 is not persistent.  S3 is a
> better solution than S4, in that S4 requires the guest to have enough
> memory (and if it doesn't cooperate, data is lost), but with S3, even if
> the guest doesn't cooperate, we can still fall back to migration to file
> with the guest only losing time, but not data.

Trying to hook into host S3/S4 and do magic to the guests is just
asking for trouble. Not only can it arbitrarily delay the host going
into S3/S4, but it is not reliable in general, even for OS which do
support it. Much better off hooking into the resume path on the host
and issuing a QEMU GA call to each running guest to resync their
clocks

Regards,
Daniel
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