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

[Jason Wang]

On 2021/2/23 1:09 上午, Si-Wei Liu wrote:
> 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?


Yes, but the problem is we can't detect whether or not it's an legacy 
guest (e.g feature is not set).


> I think the point is, since there's legacy guest we'd have to support, 
> this host side workaround is unavoidable.


Since from vhost-vDPA point of view the driver is Qemu, it means we need 
make qemu vhost-vDPA driver spec complaint.

E.g how about:

1) revert 452639a64ad8 and fix Qemu? In Qemu, during vhost-vDPA 
initialization, do a minial config sync by neogitating minimal features 
(e.g just VIRTIO_F_ACCESS_PLATFORM). When FEATURE_OK is not set from 
guest, we can only allow it to access the config space that is emulated 
by Qemu?

Then


> Although I agree the violating driver should be fixed (yes, it's in 
> today's upstream kernel which exists for a while now).


2) Fix the virtio driver bug.

Or a quick workaround is to set (VIRTIO_F_ACCESS_PLATFORM instead of 0) 
in this case.

Thanks


> -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)