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Date:   Mon, 8 Feb 2021 07:35:00 +0200
From:   Eli Cohen <elic@...dia.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>
CC:     <mst@...hat.com>, <jasowang@...hat.com>,
        <linux-kernel@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/3] mlx5_vdpa: fix feature negotiation across device
 reset

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@...cle.com>
> ---
>  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.

Did you actually see this issue in action? If you did, can you share
with us how you trigerred this?

> -- 
> 1.8.3.1
> 

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