Re: [PATCH v2 4/4] virtio-net: Add support for USO features

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Aug 08, 2024 at 07:52:37PM +0900, Akihiko Odaki wrote:
> On 2024/08/06 22:29, Michael S. Tsirkin wrote:
> > On Tue, Aug 06, 2024 at 04:35:44PM +0900, Akihiko Odaki wrote:
> > > On 2024/08/05 19:08, Michael S. Tsirkin wrote:
> > > > On Mon, Aug 05, 2024 at 06:37:58PM +0900, Akihiko Odaki wrote:
> > > > > If cross-migrate=off, QEMU can still migrate on the same host (checkpoint
> > > > > and restart). QEMU can also migrate across hosts if the user ensures they
> > > > > are on the same platform.
> > > > 
> > > > What is so special about checkpoint/restart? I guess we hope that
> > > > downgrades are uncommon, but they are possible...
> > > 
> > > Downgrades will not work with cross-migrate=off. Users who want downgrades
> > > should use cross-migrate=on.
> > 
> > We also don't know that upgrades do not disable a feature:
> > can happen if e.g. there's a serious bug in the feature.
> > Basically, this makes the feature too fragile, in my opinion.
> 
> We can do nothing in such a case. Whether it is on a single host or multiple
> hosts, we cannot support migration if features once enabled disappear.
> 
> Regards,
> Akihiko Odaki

It does not follow that we have to do something, and this is something,
therefore that we have to do this.

This is just a reason not to handle checkpoint/restart any different
than any other migration.

-- 
MST