Re: [PATCH v2] virtio_net: CTRL_GUEST_OFFLOADS depends on CTRL_VQ

[Jason Wang]

On 2020/1/6 äå8:54, Michael S. Tsirkin wrote:
> On Mon, Jan 06, 2020 at 10:47:35AM +0800, Jason Wang wrote:
> > On 2020/1/5 äå9:22, Michael S. Tsirkin wrote:
> > > The only way for guest to control offloads (as enabled by
> > > VIRTIO_NET_F_CTRL_GUEST_OFFLOADS) is by sending commands
> > > through CTRL_VQ. So it does not make sense to
> > > acknowledge VIRTIO_NET_F_CTRL_GUEST_OFFLOADS without
> > > VIRTIO_NET_F_CTRL_VQ.
> > >
> > > The spec does not outlaw devices with such a configuration, so we have
> > > to support it. Simply clear VIRTIO_NET_F_CTRL_GUEST_OFFLOADS.
> > > Note that Linux is still crashing if it tries to
> > > change the offloads when there's no control vq.
> > > That needs to be fixed by another patch.
> > >
> > > Reported-by: Alistair Delva <adelva@xxxxxxxxxx>
> > > Reported-by: Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>
> > > Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand if possible on XDP set")
> > > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > ---
> > >
> > > Same patch as v1 but update documentation so it's clear it's not
> > > enough to fix the crash.
> > >
> > >    drivers/net/virtio_net.c | 9 +++++++++
> > >    1 file changed, 9 insertions(+)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 4d7d5434cc5d..7b8805b47f0d 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -2971,6 +2971,15 @@ static int virtnet_validate(struct virtio_device *vdev)
> > >    	if (!virtnet_validate_features(vdev))
> > >    		return -EINVAL;
> > > +	/* VIRTIO_NET_F_CTRL_GUEST_OFFLOADS does not work without
> > > +	 * VIRTIO_NET_F_CTRL_VQ. Unfortunately spec forgot to
> > > +	 * specify that VIRTIO_NET_F_CTRL_GUEST_OFFLOADS depends
> > > +	 * on VIRTIO_NET_F_CTRL_VQ so devices can set the later but
> > > +	 * not the former.
> > > +	 */
> > > +	if (!virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
> > > +			__virtio_clear_bit(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS);
> >
> > If it's just because a bug of spec, should we simply fix the bug and fail
> > the negotiation in virtnet_validate_feature()?
> One man's bug is another man's feature: arguably leaving the features
> independent in the spec might allow reuse of the feature bit without
> breaking guests.
>
> And even if we say it's a bug we can't simply fix the bug in the
> spec: changing the text for a future version does not change the fact
> that devices behaving according to the spec exist.
>
> > Otherwise there would be inconsistency in handling feature dependencies for
> > ctrl vq.
> >
> > Thanks
> That's a cosmetic problem ATM. It might be a good idea to generally
> change our handling of dependencies, and clear feature bits instead of
> failing probe on a mismatch.


Something like I proposed in the past ? [1]

[1] https://lore.kernel.org/patchwork/patch/519074/


>   It's worth thinking  - at the spec level -
> how we can best make the configuration extensible.
> But that's not something spec should worry about.
>
>
> > > +
> > >    	if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
> > >    		int mtu = virtio_cread16(vdev,
> > >    					 offsetof(struct virtio_net_config,