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

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(a)redhat.com&gt; 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?