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

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Message-ID: <4d6b8fc7-e697-d027-7763-8e56e0669a31@redhat.com>
Date:   Mon, 8 Feb 2021 12:37:24 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>, mst@...hat.com, elic@...dia.com
Cc:     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 2021/2/6 下午8:29, 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 is not the behavior of Qemu but it's valid.


> 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>


Acked-by: Jason Wang <jasowang@...hat.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;
>   	}