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SubjectRe: [PATCH v2] virtio_net: CTRL_GUEST_OFFLOADS depends on CTRL_VQ
On Mon, Jan 06, 2020 at 10:47:35AM +0800, Jason Wang wrote:
>
> On 2020/1/5 下午9:22, 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@google.com>
> > Reported-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> > Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand if possible on XDP set")
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >
> > 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. 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,

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