[libvirt] [Spice-devel] [PATCH v4] qemu: Use heads parameter for QXL driver

Daniel P. Berrange berrange at redhat.com
Tue Jul 21 13:41:53 UTC 2015

On Tue, Jul 21, 2015 at 03:35:50PM +0200, Martin Kletzander wrote:
> On Tue, Jul 21, 2015 at 01:50:22PM +0100, Daniel P. Berrange wrote:
> >On Tue, Jul 21, 2015 at 11:44:27AM +0200, Martin Kletzander wrote:
> >>On Tue, Jul 21, 2015 at 09:36:55AM +0200, Christophe Fergeau wrote:
> >>>On Mon, Jul 20, 2015 at 11:25:52AM +0200, Martin Kletzander wrote:
> >>>>I spend all morning fixing this to be installed properly in the
> >>>>system.  Anyway, I finally managed to make this work and found out the
> >>>>guest I used for it is not ready to have multiple monitors.  Anyway,
> >>>>looking at everything else this definitely works, so ACK, I'll push it
> >>>>in a while.
> >>>
> >>>I'm still a bit worried that all existing guests will have heads='1' in
> >>>their XML as libvirt is adding that by default, but I don't really see a
> >>>way around it :-/ We should make sure libvirt stops adding it from now
> >>>on though ;)
> >>>
> >>
> >>Well, how would you then limit the outputs to 1?  Having said that, I
> >>have no idea why we started adding heads="1" automatically and playing
> >>with different changes, we have another bug in the parsing/formatting
> >>code.  You'll also won't be able to migrate from older libvirt with
> >>heads='1' to newer ones.  I haven't realized that.  I'm thinking about
> >>reverting the change in libvirt or even using heads > 1.  Although
> >>that won't migrate either.  So the only other thing that we can do is
> >>to introduce new parameter, like maxHeads.  The sound you just heard
> >>is me slapping my face because again there will multiple parameters
> >>meaning similar things...
> >
> >Introducing a new parameter is not really viable IMHO, because as you
> >say it'll leave us with two things meaning the same, and will not be
> >compatible with existing apps that know about the current parameter.
> >
> >I think we need to figure out a way to drop the 'heads=1' from any
> >existing configs in /etc/libvirt/qemu when we start up with the new
> >libvirtd for the first time.
> >
> I'm already working on an RFC that would differentiate between loading
> XMLs on daemon start-up and defining them.  The purpose of that is so
> that we can make incompatible XML changes without losing any domains,
> but that's not the point here.  If we were to drop heads='1' anywhere,
> this is the place.  The ABI change would be minimal for the guest.

It isn't sufficient to just differentiate loading on startup
vs defining them IIUC. We need to differentiate loading on
startup from XML previously written by a libvirtd < X.Y.Z
which is why I think we need to have a version number recorded
in the XML ultimately.

> >To make this work we probably need to write a magic attribute or
> >comment into the XML configs to record which version of libvirt
> >saved it to disk. That way we know whether this is the first time
> >loading the config with the new libvirt. It will help us with
> >similar situations in the future. In this case, we'd just look
> >to see if the libvirt version number was missing from the XML,
> >and if so, then drop heads=1. If a version number is present, then
> >we're new enough, so we can keep it.  We'd also ned to pass this
> >version number in the XML when migrating, or possibly via the
> >migration cookie.
> >
> And you're suggesting to differentiate this by a comment?  That's
> really, *really* hacky.  Anyway, do you agree on temporarily reverting
> the commit so we don't release another version with it?  The changes
> will also be clearer when they will not modify what this commit
> changed.

Actually we'd need to have a formal attribute to deal with the migration
case, so slightly less hacky.

Yeah, if we can't immediately fix it, we should revert it until we have
a decent fix without regression. Likewise probably fix it on stable
branch too.

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