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

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

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.

> > 
> > 
> > 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