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

[Si-Wei Liu]

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@xxxxxxxxxx>
> > ---
> >   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)