netdev - Re: [PATCH v2] virtio_net: CTRL_GUEST_OFFLOADS depends on CTRL

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Date:   Tue, 7 Jan 2020 10:29:08 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Alistair Delva <adelva@...gle.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2] virtio_net: CTRL_GUEST_OFFLOADS depends on CTRL_VQ


On 2020/1/6 下午8:54, Michael S. Tsirkin wrote:
> 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@...gle.com>
>>> Reported-by: Willem de Bruijn <willemdebruijn.kernel@...il.com>
>>> Fixes: 3f93522ffab2 ("virtio-net: switch off offloads on demand if possible on XDP set")
>>> Signed-off-by: Michael S. Tsirkin <mst@...hat.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.


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,