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Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
To:     Si-Wei Liu <si-wei.liu@oracle.com>, mst@redhat.com, elic@nvidia.com
Cc:     linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org
References: <1613735698-3328-1-git-send-email-si-wei.liu@oracle.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <605e7d2d-4f27-9688-17a8-d57191752ee7@redhat.com>
Date:   Mon, 22 Feb 2021 12:14:17 +0800
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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?

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)