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Re: [libvirt] [PATCH v5 0/2] qemu: Support for QXL heads



On Fri, May 06, 2016 at 01:27:15PM +0200, Pavel Hrdina wrote:
> On Thu, May 05, 2016 at 07:18:51PM -0400, John Ferlan wrote: 
> 
> [...]
> 
> > Been sitting on list for a while ....
> > 
> > Obviously I think you know you have to update to top of tree
> > 
> > Would be nice to perhaps add a few intro comments to
> > qemuMigratePrepareDomain at least with respect to what the purpose is
> > and what "could" be done in the future in the code.  Looks like to me
> > it's now a shim to qemuProcessPrepareDomain to take care of any of those
> > "inconsistencies" between what can be supported in/on the new system
> > (perhaps could work in the opposite direction too ;-))... When I'm
> > reading code, I'm not necessarily looking at the commit message that
> > added which may have that information.
> > 
> > I think you could also update the commit message to point at the
> > previous code that was reverted to help understand the history.
> > 
> > ACK for the concept - looks like things are OK to me.
> > 
> > John
> 
> The only issue with this patch is that it breaks migration back to older
> libvirt.  We generally try to not break migration to old libvirt if you migrate
> from new libvirt to old libvirt with the same XML that would be also valid for
> the old libvirt.  Since there is no change in the XML and we start using the
> 'heads' attribute and we now pass that value to qemu you cannot migrate back to
> some older libvirt.
> 
> NACK, we need to figure this out.

Yep, the major use of QXL is RHEV/oVirt and they explicitly require libvirt
to support migration to old versions. So we need to figure this out.

Regards,
Daniel
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