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

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.