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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 23 Feb 2021 07:26:28 -0500

On Fri, Feb 19, 2021 at 06:54:58AM -0500, 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. 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")

isn't this more

    vdpa: make sure set_features is invoked for legacy

> Signed-off-by: Si-Wei Liu <[email-masked]>

I think we at least need to correct the comment in
include/linux/vdpa.h then

Instead of "we assume a legacy guest" we'd say something like
"call set features in case it's a legacy guest".

Generally it's unfortunate. Need to think about what to do here.
Any idea how else we can cleanly detect a legacy guest?

> ---
>  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;

Let's just set VIRTIO_F_ACCESS_PLATFORM in core?
Then we don't need to hack mlx5 ...

> @@ -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)
> -- 
> 1.8.3.1