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Re: [libvirt] [PATCH] qemu driver: sync guest time via qemu-guest-agent when domain is resumed



On 11.02.2014 01:04, Marcelo Tosatti wrote:
On Mon, Feb 10, 2014 at 03:42:45PM -0700, Eric Blake wrote:
On 02/10/2014 03:10 PM, Marcelo Tosatti wrote:

Since QEMU commit "kvmclock: clock should count only if vm is running"
(http://lists.gnu.org/archive/html/qemu-devel/2013-06/msg01225.html),
guests have their realtime clock stopped during pause.

To correct the situation, invoke guest agent to sync time from
host time.

This needs to be conditional on new API (either a flag to existing API
[except that virDomainResumeFlags is not quite existing yet], or new API
altogether).

We cannot require an API to depend on guest interaction (which an agent
command does) without an explicit flag mentioning that it is okay.  We
may even decide to have conditional ACL checks (allowing some users the
ability to restart a domain, but not to interact with the guest agent).

For that matter, when we proposed adding virDomainResumeFlags as the way
to add in a flag for requesting a sync via this API, Dan suggested it
would be better to add a new API altogether instead of piggybacking on
top of the existing API:

https://www.redhat.com/archives/libvir-list/2014-February/msg00104.html

"So there's interesting, and I'm not sure I entirely agree about
adding flags here. It seems to me that if the QEMU agent has a
"set-time" capability we'd be better off having an explicit API
virDomainSetTime(...).  The action of resume + set time cannot
be atomic so I don't see any point in overloading the "set time"
functionality into the resume API call. Just let apps call the
set time method if they so desire."

Apps desire the guest time to be synchronized to host time, which is the
standard 99% of usecases configuration.

Ok, i was thinking of adding a new element GuestSyncTimeOnResume to
control this, which would default to on (as users expect their guests
time to be correct at all times, without additional commands, and
rightly so).

It seems unreasonable to me that every API user has to keep track of
every location which a guest can possibly resume execution from a paused
state: this can be hidden inside libvirt.

Daniel, all the logic behind the necessity to set-time after guest
resume (*) can be hidden inside libvirt.

(*) all instances of guest resume: upon migration continuation, upon
ENOSPACE pause continuation, all of them.

Don't see the need to force the knowledge and maintenance of this state
on libvirt users.

Are you OK with the new knob, default on, to control sync time on
guest resume, then?



I think we may need both approaches. I too think that resume with syncing guest time is so common use case that is deserves to be exposed as a single operation outside the libvirt.

On the other hand, we certainly want to expose this as a new virDomain{Get,Set}Time() API.

BTW: I would have implement this already, but I'm still waiting for my qemu patch to be merged upstream:

http://lists.nongnu.org/archive/html/qemu-devel/2014-01/msg04293.html

I don't really want to start implementing this feature with need to patch qemu myself.

Michal


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