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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 23 Feb 2021 04:26:35 -0500

On Mon, Feb 22, 2021 at 08:05:26AM +0200, Eli Cohen wrote:
> On Sun, Feb 21, 2021 at 04:52:05PM -0500, Michael S. Tsirkin wrote:
> > 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 <[email-masked]>
> > > > ---
> > > >  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"?
> > 
> 
> Actually I meant this driver/device does not support such cases.

Well the removed code merely failed without VIRTIO_F_ACCESS_PLATFORM
it didn't actually try to support anything ...

> > 
> > > 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
> > > > 
> >