Re: [PATCH 2/3] mlx5_vdpa: fix feature negotiation across device reset

[Si-Wei Liu]

On 2/7/2021 9:35 PM, Eli Cohen wrote:
> On Sat, Feb 06, 2021 at 04:29:23AM -0800, Si-Wei Liu wrote:
> > The mlx_features denotes the capability for which
> > set of virtio features is supported by device. In
> > principle, this field needs not be cleared during
> > virtio device reset, as this capability is static
> > and does not change across reset.
> >
> > In fact, the current code may have the assumption
> > that mlx_features can be reloaded from firmware
> > via the .get_features ops after device is reset
> > (via the .set_status ops), which is unfortunately
> > not true. The userspace VMM might save a copy
> > of backend capable features and won't call into
> > kernel again to get it on reset. This causes all
> > virtio features getting disabled on newly created
> > virtqs after device reset, while guest would hold
> > mismatched view of available features. For e.g.,
> > the guest may still assume tx checksum offload
> > is available after reset and feature negotiation,
> > causing frames with bogus (incomplete) checksum
> > transmitted on the wire.
> >
> > Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
> > ---
> >   drivers/vdpa/mlx5/net/mlx5_vnet.c | 1 -
> >   1 file changed, 1 deletion(-)
> >
> > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > index b8416c4..aa6f8cd 100644
> > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > @@ -1788,7 +1788,6 @@ static void mlx5_vdpa_set_status(struct vdpa_device *vdev, u8 status)
> >   		clear_virtqueues(ndev);
> >   		mlx5_vdpa_destroy_mr(&ndev->mvdev);
> >   		ndev->mvdev.status = 0;
> > -		ndev->mvdev.mlx_features = 0;
> >   		++mvdev->generation;
> >   		return;
> >   	}
> Since we assume that device capabilities don't change, I think I would
> get the features through a call done in mlx5v_probe after the netdev
> object is created and change mlx5_vdpa_get_features() to just return
> ndev->mvdev.mlx_features.
Yep, it makes sense. Will post a revised patch. If vdpa tool allows 
reconfiguration post probing, the code has to be reconciled then.

> Did you actually see this issue in action? If you did, can you share
> with us how you trigerred this?
Issue is indeed seen in action. The mismatched tx-checksum offload as 
described in the commit message was one of such examples. You would need 
a guest reboot though (triggering device reset via the .set_status ops 
and zero'ed mlx_features was loaded to deduce new actual_features for 
creating the h/w virtq object) for repro.

-Siwei
> > --
> > 1.8.3.1