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

[Jason Wang <jasowang@redhat.com>]

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()?

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,