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

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

On Tue, Apr 22, 2025 at 07:49:41PM +0200, Paolo Abeni wrote:
> On 1/26/25 7:20 AM, Parav Pandit wrote:
> > @@ -136,6 +124,18 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
> >  \item[VIRTIO_NET_F_SPEED_DUPLEX(63)] Device reports speed and duplex.
> >  
> >  \item[VIRTIO_NET_F_RSS_CONTEXT(64)] Device supports multiple RSS contexts.
> > +
> > +\item[VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO (65)] Driver can receive GSO packets
> > +  carried by a UDP tunnel.
> > +
> > +\item[VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_CSUM (66)] Driver handles packets
> > +  carried by a UDP tunnel with partial csum for the outer header.
> > +
> > +\item[VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO (67)] Device can receive GSO packets
> > +  carried by a UDP tunnel.
> > +
> > +\item[VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO_CSUM (68)] Device handles packets
> > +  carried by a UDP tunnel with partial csum for the outer header.
> >  \end{description}
> 
> I'm finally trying to finalize the implementation and I must admit I
> underlooked the implication of using features bit >= 64.
> 
> AFAICS all the virtio drivers in the Linux kernel and qemu assume that
> the features can be represented with a single u64.
> 
> Supporting the above requires major reworks in both user-space and
> kernel space. At least on the kernel side a similar rework (extending
> the _netdev_ features field) has proven to be complex enough to defeat
> any implementation attempts for years and in the end it was dismissed in
> favor of lower bits reuse.
> 
> I'm wondering if we could reconsider using reserved bits here?
> 
> Thanks!
> 
> Paolo

:(

It's been there for 2 months now.

And if you want to avoid using all bits > 63 then it's even more of
a mess, you need to move VIRTIO_NET_F_RSS_CONTEXT which has been
there for a year and a half.

I'm afraid we'll have to bite the bullet.

-- 
MST