Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero

[Si-Wei Liu <si-wei.liu@oracle.com>]

On 2/21/2021 8:14 PM, Jason Wang wrote:
>
> On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
>> Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
>> for legacy") made an exception for legacy guests to reset
>> features to 0, when config space is accessed before features
>> are set. We should relieve the verify_min_features() check
>> and allow features reset to 0 for this case.
>>
>> It's worth noting that not just legacy guests could access
>> config space before features are set. For instance, when
>> feature VIRTIO_NET_F_MTU is advertised some modern driver
>> will try to access and validate the MTU present in the config
>> space before virtio features are set.
>
>
> This looks like a spec violation:
>
> "
>
> The following driver-read-only field, mtu only exists if 
> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the 
> driver to use.
> "
>
> Do we really want to workaround this?

Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?

I think the point is, since there's legacy guest we'd have to support, 
this host side workaround is unavoidable. Although I agree the violating 
driver should be fixed (yes, it's in today's upstream kernel which 
exists for a while now).

-Siwei

>
> Thanks
>
>
>> Rejecting reset to 0
>> prematurely causes correct MTU and link status unable to load
>> for the very first config space access, rendering issues like
>> guest showing inaccurate MTU value, or failure to reject
>> out-of-range MTU.
>>
>> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 
>> devices")
>> Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
>> ---
>>   drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
>>   1 file changed, 1 insertion(+), 14 deletions(-)
>>
>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c 
>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>> index 7c1f789..540dd67 100644
>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>> @@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct 
>> vdpa_device *vdev)
>>       return mvdev->mlx_features;
>>   }
>>   -static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 
>> features)
>> -{
>> -    if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
>> -        return -EOPNOTSUPP;
>> -
>> -    return 0;
>> -}
>> -
>>   static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
>>   {
>>       int err;
>> @@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct 
>> vdpa_device *vdev, u64 features)
>>   {
>>       struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>       struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>> -    int err;
>>         print_features(mvdev, features, true);
>>   -    err = verify_min_features(mvdev, features);
>> -    if (err)
>> -        return err;
>> -
>>       ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
>>       ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
>>       ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, 
>> VIRTIO_NET_S_LINK_UP);
>> -    return err;
>> +    return 0;
>>   }
>>     static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, 
>> struct vdpa_callback *cb)
>