Re: [Qemu-devel] [PATCH V3] hw/virtio: Add PCIe capability to virtio devices

[Eduardo Habkost]

On Mon, Nov 02, 2015 at 02:12:32PM +0200, Marcel Apfelbaum wrote:
> On 11/02/2015 02:05 PM, Cornelia Huck wrote:
[...]
> >What's the word on compat machines and new device types, btw.? If we
> >fire up a compat machine, we can still specify devices that were added
> >with later machine versions, but of course we can't migrate to an old
> >machine as the device types did not exist there. Do we want to give the
> >user a hint here by disallowing new device types?
> >
> 
> I started to wonder about this too, so I added to this thread the migration
> maintainers that should be qualified to answer this :)

This looks no different from all other features that are available on
newer QEMU versions and prevent migration to older hosts (even ones that
are not guest-visible, like backend configuration). Management can
easily detect the unavailability of those features in other hosts, long
before trying migration (and have better ways to warn the user if
necessary).

Also, it looks like a potential nightmare for downstream maintainers
that cherry-pick and rebase patches, so I hope we don't consider
implementing that. :)

-- 
Eduardo