[libvirt PATCH 4/4] qemu: Enable migration events only for fresh QEMU process

[Peter Krempa]

On Thu, May 12, 2022 at 13:30:57 +0200, Jiri Denemark wrote:
> On Thu, May 12, 2022 at 12:41:57 +0200, Peter Krempa wrote:
> > On Thu, May 12, 2022 at 12:40:54 +0200, Peter Krempa wrote:
> > > On Thu, May 12, 2022 at 10:55:47 +0200, Jiri Denemark wrote:
> > > > Every running QEMU process we are willing to reconnect (i.e., at least
> > > > 3.1.0) supports migration events and we can assume the capability is
> > > > already enabled since last time libvirt daemon connected to its monitor.
> > > > 
> > > > Well, it's not guaranteed though. If libvirt 1.2.17 or older was used to
> > > > start QEMU 3.1.0 or newer, migration events would not be enabled. And if
> > > > the user decides to upgrade libvirt from 1.2.17 to 8.4.0 while the QEMU
> > > > process is still running, they would not be able to migrate the domain
> > > 
> > > But doesn't the function below actually enable the events? Or is there
> > > something else that needs to be done?
> > > 
> > > Such that we simply could enable the events and be done with it?
> > > 
> > > > because of disabled migration events. I think we do not really need to
> > > > worry about this scenario as libvirt 1.2.17 is 7 years old while QEMU
> > > > 3.1.0 was released only 3.5 years ago. Thus a chance someone would be
> > > > running such configuration should be fairly small and a combination with
> > > > upgrading 1.2.17 to 8.4.0 (or newer) with running domains should get it
> > > > pretty much to zero. The issue would disappear ff the ancient libvirt is
> > > > first upgraded to something older than 8.4.0 and then to the current
> > > > libvirt.
> > > > 
> > > > Signed-off-by: Jiri Denemark <jdenemar at redhat.com>
> > > > ---
> > > > 
> > > > Notes:
> > > >     I was hoping we could do this without any downside, but it appeared this
> > > >     was not possible. In case someone still thinks this would be an issue, I
> > > >     can take an alternative solution and check whether migration events are
> > > >     enabled when reconnecting to QEMU monitor.
> > 
> > Aaah, never mind. You want to avoid setting it all the time. Well I
> > think I would be okay with that, does our code handle the absence of
> > events properly?
> 
> I guess the migration would just hang waiting for an event that never
> comes. But I guess it should be fairly easy to check whether events are
> enabled before starting a migration and report an error instead of
> hanging.

Leaving the VM in a hanging migration would be bad, but if we can just
refuse it I'd be okay with that.