Re: [PATCH v1] virtio-net: Fix to avoid using reserved feature bits

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

On Mon, Apr 28, 2025 at 07:07:12PM +0200, Paolo Abeni wrote:
> Hi,
> 
> I'll try to consolidate here the current discussion.
> 
> My understanding is that the we are on same WRT that there is a problem
> with the current spec.
> 
> On 4/28/25 11:13 AM, Michael S. Tsirkin wrote:
> > Given features are negotiated first, we can just say the value
> > can be 64 or 128 bits? In any case, I agree it's buggy as defined.
> 
> Do you mean that the 'offload' field should be 64 or 128 bits long
> depending on the negotiated features?
> 
> Could that be problematic with VM migrations and/or backward
> compatibility? I can't see a specific faulty scenario, but the 'implied'
> field size sounds fragile to me.
> 
> /P

My idea is to say any number of le64 values, and any bits excluded
must be 0.

So looking at qemu for example, if you send it more than 64 bit
it will just ignore the top 64 bit. But it does not currently
support any low bits, so it's fine.

Again any change will break compat technicall but then
any change to reserved bits does, too.


-- 
MST