Re: [RFC PATCH] virtio-net: introduce strict peer feature check

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

On Thu, Nov 13, 2025 at 12:12:38PM -0500, Peter Xu wrote:
> On Thu, Nov 13, 2025 at 11:47:51AM -0500, Michael S. Tsirkin wrote:
> > On Thu, Nov 13, 2025 at 11:37:25AM -0500, Peter Xu wrote:
> > > On Thu, Nov 13, 2025 at 11:09:32AM -0500, Michael S. Tsirkin wrote:
> > > > On Fri, Nov 07, 2025 at 10:01:49AM +0800, Jason Wang wrote:
> > > > > We used to clear features silently in virtio_net_get_features() even
> > > > > if it is required. This complicates the live migration compatibility
> > > > > as the management layer may think the feature is enabled but in fact
> > > > > not.
> > > > > 
> > > > > Let's add a strict feature check to make sure if there's a mismatch
> > > > > between the required feature and peer, fail the get_features()
> > > > > immediately instead of waiting until the migration to fail. This
> > > > > offload the migration compatibility completely to the management
> > > > > layer.
> > > > > 
> > > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > > 
> > > > This is not really useful - how do users know how to tweak their
> > > > command lines?
> > > > We discussed this many times.
> > > > To try and solve this you need a tool that will tell you how to start
> > > > VM on X to make it migrateable to Y or Z.
> > > > 
> > > > 
> > > > More importantly,
> > > > migration is a niche thing and breaking booting perfectly good VMs 
> > > > just for that seems wrong.
> > > 
> > > IMHO Jason's proposal is useful in that it now provides a way to provide
> > > ABI stablility but allows auto-ON to exist.
> > > 
> > > If we think migration is optional, we could add a migration blocker where
> > > strict check flag is set to OFF, as I mentioned in the email reply to Dan.
> > > As that implies the VM ABI is not guaranteed.
> > > 
> > > Thanks,
> > 
> > 
> > All you have to do is avoid changing the kernel and ABI is stable.
> > Downstreams already do this.
> 
> But the whole point of migration is allowing VMs to move between hosts..
> hence AFAIU kernel can change.
> 
> Downstream will still have problem if some network features will be
> optionally supported in some of the RHEL-N branches, because machine types
> are defined the same in any RHEL-N, so IIUC it's also possible a VM booting
> on a latest RHEL-X.Y qemu/kernel hit issues migrating back to an older
> RHEL-X.(Y-1) qemu/kernel if RHEL-X.(Y-1) kernel doesn't have the network
> feature available..
> 
> It's also not good IMHO to only fix downstream but having upstream face
> such problems, even if there's a downstream fix...
> 
> This thread was revived only because Jinpu hit similar issues.  IMHO we
> should still try to provide a generic solution upstream for everyone.
> 
> Thanks,
> 
> -- 
> Peter Xu

failing to start a perfectly good qemu which used to work
because you changed kernels is better than failing to migrate how?



graceful downgrade with old kernels is the basics of good userspace
behaviour and has been for decades.


sure, let's work on a solution, just erroring out is more about blaming
the user. what is the user supposed to do when qemu fails to start?


first, formulate what exactly do you want to enable.



for example, you have a set of boxes and you want a set of flags
to supply to guarantee qemu can migrate between them. is that it?



-- 
MST