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

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Date:   Tue, 23 Feb 2021 04:55:37 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Eli Cohen <elic@...dia.com>, Si-Wei Liu <si-wei.liu@...cle.com>,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero

On Tue, Feb 23, 2021 at 05:48:10PM +0800, Jason Wang wrote:
> 
> On 2021/2/23 下午5:26, Michael S. Tsirkin wrote:
> > 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<si-wei.liu@...cle.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"?
> > > > 
> > > 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 ...
> 
> 
> I think it's used to catch the driver that doesn't support ACCESS_PLATFORM?
> 
> Thanks
> 

That is why I asked whether Eli meant catch.

-- 
MST