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

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Date:   Tue, 7 Jan 2020 16:31:14 +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 2020/1/7 下午3:06, Michael S. Tsirkin wrote:
> On Tue, Jan 07, 2020 at 10:29:08AM +0800, Jason Wang wrote:
>> 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/
>
> No that still fails probe.
>
> I am asking whether it's more future proof to fail probe
> on feature combinations disallowed by spec, or to clear bits
> to get to an expected combination.


Sorry wrong link.

It should be: https://lkml.org/lkml/2014/11/17/82


>
> In any case, we should probably document in the spec how
> drivers behave on such combinations.


Yes.

Thanks


>
>
>>>    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,