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

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

On Sun, Aug 18, 2024 at 02:04:29PM +0900, Akihiko Odaki wrote:
> On 2024/08/09 21:50, Fabiano Rosas wrote:
> > Peter Xu <peterx@redhat.com> writes:
> > 
> > > On Thu, Aug 08, 2024 at 10:47:28AM -0400, Michael S. Tsirkin wrote:
> > > > On Thu, Aug 08, 2024 at 10:15:36AM -0400, Peter Xu wrote:
> > > > > On Thu, Aug 08, 2024 at 07:12:14AM -0400, Michael S. Tsirkin wrote:
> > > > > > This is too big of a hammer. People already use what you call "cross
> > > > > > migrate" and have for years. We are not going to stop developing
> > > > > > features just because someone suddenly became aware of some such bit.
> > > > > > If you care, you will have to work to solve the problem properly -
> > > > > > nacking half baked hacks is the only tool maintainers have to make
> > > > > > people work on hard problems.
> > > > > 
> > > > > IMHO this is totally different thing.  It's not about proposing a new
> > > > > feature yet so far, it's about how we should fix a breakage first.
> > > > > 
> > > > > And that's why I think we should fix it even in the simple way first, then
> > > > > we consider anything more benefitial from perf side without breaking
> > > > > anything, which should be on top of that.
> > > > > 
> > > > > Thanks,
> > > > 
> > > > As I said, once the quick hack is merged people stop caring.
> > > 
> > > IMHO it's not a hack. It's a proper fix to me to disable it by default for
> > > now.
> > > 
> > > OTOH, having it ON always even knowing it can break migration is a hack to
> > > me, when we don't have anything else to guard the migration.
> > > 
> > > > Mixing different kernel versions in migration is esoteric enough for
> > > > this not to matter to most people. There's no rush I think, address
> > > > it properly.
> > > 
> > > Exactly mixing kernel versions will be tricky to users to identify, but
> > > that's, AFAICT, exactly happening everywhere.  We can't urge user to always
> > > use the exact same kernels when we're talking about a VM cluster.  That's
> > > why I think allowing migration to work across those kernels matter.
> > 
> > I also worry a bit about the scenario where the cluster changes slightly
> > and now all VMs are already restricted by some option that requires the
> > exact same kernel. Specifically, kernel changes in a cloud environment
> > also happen due to factors completely unrelated to migration. I'm not
> > sure the people managing the infra (who care about migration) will be
> > gating kernel changes just because QEMU has been configured in a
> > specific manner.
> 
> I have wrote a bit about the expectation on the platform earlier[1], but let
> me summarize it here.
> 
> 1. I expect the user will not downgrade the platform of hosts after setting
> up a VM. This is essential to enable any platform feature.
> 
> 2. The user is allowed to upgrade the platform of hosts gradually. This
> results in a situation with mixed platforms. The oldest platform is still
> not older than the platform the VM is set up for. This enables the gradual
> deployment strategy.
> 
> 3. the user is allowed to downgrade the platform of hosts to the version
> used when setting up the VM. This enables rollbacks in case of regression.
> 
> With these expectations, we can ensure migratability by a) enabling platform
> features available on all hosts when setting up the VM and b) saving the
> enabled features. This is covered with my
> -dump-platform/-merge-platform/-use-platform proposal[2].

I really like [2]. Do you plan to work on it? Does anyone else?

> Regards,
> Akihiko Odaki
> 
> [1]
> https://lore.kernel.org/r/2b62780c-a6cb-4262-beb5-81d54c14f545@daynix.com
> [2]
> https://lore.kernel.org/all/2da4ebcd-2058-49c3-a4ec-8e60536e5cbb@daynix.com/