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

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Feb 21, 2021 at 04:44:37PM +0200, Eli Cohen wrote:
> 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")
> > 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;
> > -}
> > -
> 
> But what if VIRTIO_F_ACCESS_PLATFORM is not offerred? This does not
> support such cases.

Did you mean "catch such cases" rather than "support"?


> Maybe we should call verify_min_features() from mlx5_vdpa_set_status()
> just before attempting to call setup_driver().
> 
> >  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)
> > -- 
> > 1.8.3.1
> >