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

[Jason Wang <jasowang@redhat.com>]

On 2019/12/24 上午4:21, Alistair Delva wrote:
> On Mon, Dec 23, 2019 at 12:12 PM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
>> On Mon, Dec 23, 2019 at 2:56 PM Willem de Bruijn
>> <willemdebruijn.kernel@gmail.com> wrote:
>>> 00fffe0ff0 DR7: 0000000000000400
>>>>> Call Trace:
>>>>>   ? preempt_count_add+0x58/0xb0
>>>>>   ? _raw_spin_lock_irqsave+0x36/0x70
>>>>>   ? _raw_spin_unlock_irqrestore+0x1a/0x40
>>>>>   ? __wake_up+0x70/0x190
>>>>>   virtnet_set_features+0x90/0xf0 [virtio_net]
>>>>>   __netdev_update_features+0x271/0x980
>>>>>   ? nlmsg_notify+0x5b/0xa0
>>>>>   dev_disable_lro+0x2b/0x190
>>>>>   ? inet_netconf_notify_devconf+0xe2/0x120
>>>>>   devinet_sysctl_forward+0x176/0x1e0
>>>>>   proc_sys_call_handler+0x1f0/0x250
>>>>>   proc_sys_write+0xf/0x20
>>>>>   __vfs_write+0x3e/0x190
>>>>>   ? __sb_start_write+0x6d/0xd0
>>>>>   vfs_write+0xd3/0x190
>>>>>   ksys_write+0x68/0xd0
>>>>>   __ia32_sys_write+0x14/0x20
>>>>>   do_fast_syscall_32+0x86/0xe0
>>>>>   entry_SYSENTER_compat+0x7c/0x8e
>>>>>
>>>>> A similar crash will likely trigger when enabling XDP.
>>>>>
>>>>> 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>
>>>>> ---
>>>>>
>>>>> Lightly tested.
>>>>>
>>>>> Alistair, could you please test and confirm that this resolves the
>>>>> crash for you?
>>>> This patch doesn't work. The reason is that NETIF_F_LRO is also turned
>>>> on by TSO4/TSO6, which your patch didn't check for. So it ends up
>>>> going through the same path and crashing in the same way.
>>>>
>>>>          if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) ||
>>>>              virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6))
>>>>                  dev->features |= NETIF_F_LRO;
>>>>
>>>> It sounds like this patch is fixing something slightly differently to
>>>> my patch fixed. virtnet_set_features() doesn't care about
>>>> GUEST_OFFLOADS, it only tests against NETIF_F_LRO. Even if "offloads"
>>>> is zero, it will call virtnet_set_guest_offloads(), which triggers the
>>>> crash.
>>>
>>> Interesting. It's surprising that it is trying to configure a flag
>>> that is not configurable, i.e., absent from dev->hw_features
>>> after Michael's change.
>>>
>>>> So either we need to ensure NETIF_F_LRO is never set, or
>>> LRO might be available, just not configurable. Indeed this was what I
>>> observed in the past.
>> dev_disable_lro expects that NETIF_F_LRO is always configurable. Which
>> I guess is a reasonable assumption, just not necessarily the case in
>> virtio_net.
>>
>> So I think we need both patches. Correctly mark the feature as fixed
>> by removing from dev->hw_features and also ignore the request from
>> dev_disable_lro, which does not check for this.
> Something like this maybe:
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4d7d5434cc5d..0556f42b0fb5 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2560,6 +2560,9 @@ static int virtnet_set_features(struct net_device *dev,
>          u64 offloads;
>          int err;
>
> +       if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS))
> +               return 0;
> +
>          if ((dev->features ^ features) & NETIF_F_LRO) {
>                  if (vi->xdp_queue_pairs)
>                          return -EBUSY;
> @@ -2971,6 +2974,15 @@ 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. However the virtio spec does not
> +        * 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,
>

We check feature dependency and fail the probe in 
virtnet_validate_features().

Is it more straightforward to fail the probe there when 
CTRL_GUEST_OFFLOADS was set but CTRL_VQ wasn't?

Thanks