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

[Peter Xu <peterx@redhat.com>]

On Fri, Jul 26, 2024 at 03:25:31AM -0400, Michael S. Tsirkin wrote:
> On Fri, Jul 26, 2024 at 09:03:24AM +0200, Thomas Huth wrote:
> > On 26/07/2024 08.08, Michael S. Tsirkin wrote:
> > > On Thu, Jul 25, 2024 at 06:18:20PM -0400, Peter Xu wrote:
> > > > On Tue, Aug 01, 2023 at 01:31:48AM +0300, Yuri Benditovich wrote:
> > > > > USO features of virtio-net device depend on kernel ability
> > > > > to support them, for backward compatibility by default the
> > > > > features are disabled on 8.0 and earlier.
> > > > > 
> > > > > Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>
> > > > > Signed-off-by: Andrew Melnychecnko <andrew@daynix.com>
> > > > 
> > > > Looks like this patch broke migration when the VM starts on a host that has
> > > > USO supported, to another host that doesn't..
> > > 
> > > This was always the case with all offloads. The answer at the moment is,
> > > don't do this.
> > 
> > May I ask for my understanding:
> > "don't do this" = don't automatically enable/disable virtio features in QEMU
> > depending on host kernel features, or "don't do this" = don't try to migrate
> > between machines that have different host kernel features?
> 
> The later.

The question is how should an user know a migration is not supported?

The user can be using exactly the same QEMU binary on two hosts, while
there can be a tiny slight difference in host kernel version, then
migration can fail between them misterously.

There're too many kernel features that can be on/off when kernels are
different, even if slightly.  Then I don't see how someone can even
identify such issue, unless one uses exactly the same host kernels on both
sides..

-- 
Peter Xu