Linux Virtualization - Jan 2020 - [PATCH v2] virtio_net: CTRL_GUEST_OFFLOADS depends on CTRL_VQ

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 at google.com>
Reported-by: Willem de Bruijn <willemdebruijn.kernel at gmail.com>
Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand if possible
on XDP set")
Signed-off-by: Michael S. Tsirkin <mst at 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 (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
 		int mtu = virtio_cread16(vdev,
 					 offsetof(struct virtio_net_config,
-- 
MST

On 2020/1/5 ??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 at google.com>
> Reported-by: Willem de Bruijn <willemdebruijn.kernel at gmail.com>
> Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand if
possible on XDP set")
> Signed-off-by: Michael S. Tsirkin <mst at 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()?

Otherwise there would be inconsistency in handling feature dependencies 
for ctrl vq.

Thanks

> +
>   	if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>   		int mtu = virtio_cread16(vdev,
>   					 offsetof(struct virtio_net_config,

On Mon, Jan 06, 2020 at 10:47:35AM +0800, Jason Wang
wrote:
> 
> On 2020/1/5 ??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 at google.com>
> > Reported-by: Willem de Bruijn <willemdebruijn.kernel at
gmail.com>
> > Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand
if possible on XDP set")
> > Signed-off-by: Michael S. Tsirkin <mst at 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,