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

[Peter Xu <peterx@redhat.com>]

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,

> 
> 
> If you want to keep this off by default, and have management
> enable this if it knows what it's doing, then I don't really
> care.

-- 
Peter Xu