Re: [RFC PATCH net] virtio_net: Prevent napi_weight changes with VIRTIO_NET_F_NOTF_COAL support

[Xuan Zhuo]

On Mon, 5 Jun 2023 14:02:36 -0700, Brett Creeley <brett.creeley@xxxxxxx> wrote:
> Commit 699b045a8e43 ("net: virtio_net: notifications coalescing
> support") added support for VIRTIO_NET_F_NOTF_COAL. The get_coalesce
> call made changes to report "1" in tx_max_coalesced_frames if
> VIRTIO_NET_F_NOTF_COAL is not supported and napi.weight is non-zero.
> However, the napi_weight value could still be changed by the
> set_coalesce call regardless of whether or not the device supports
> VIRTIO_NET_F_NOTF_COAL.
>
> It seems like the tx_max_coalesced_frames value should not control more
> than 1 thing (i.e. napi_weight and the device's tx_max_packets). So, fix
> this by only allowing the napi_weight change if VIRTIO_NET_F_NOTF_COAL
> is not supported by the virtio device.


@Jason I wonder should we keep this function to change the napi weight by the
coalesec command.

Thanks.

>
> It wasn't clear to me if this was the intended behavior, so that's why
> I'm sending this as an RFC patch initially. Based on the feedback, I
> will resubmit as an official patch.
>
> Fixes: 699b045a8e43 ("net: virtio_net: notifications coalescing support")
> Signed-off-by: Brett Creeley <brett.creeley@xxxxxxx>
> ---
>  drivers/net/virtio_net.c | 24 +++++++++++++-----------
>  1 file changed, 13 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 486b5849033d..e28387866909 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2990,19 +2990,21 @@ static int virtnet_set_coalesce(struct net_device *dev,
>  	int ret, i, napi_weight;
>  	bool update_napi = false;
>
> -	/* Can't change NAPI weight if the link is up */
> -	napi_weight = ec->tx_max_coalesced_frames ? NAPI_POLL_WEIGHT : 0;
> -	if (napi_weight ^ vi->sq[0].napi.weight) {
> -		if (dev->flags & IFF_UP)
> -			return -EBUSY;
> -		else
> -			update_napi = true;
> -	}
> -
> -	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_NOTF_COAL))
> +	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_NOTF_COAL)) {
>  		ret = virtnet_send_notf_coal_cmds(vi, ec);
> -	else
> +	} else {
> +		/* Can't change NAPI weight if the link is up */
> +		napi_weight = ec->tx_max_coalesced_frames ?
> +			NAPI_POLL_WEIGHT : 0;
> +		if (napi_weight ^ vi->sq[0].napi.weight) {
> +			if (dev->flags & IFF_UP)
> +				return -EBUSY;
> +			else
> +				update_napi = true;
> +		}
> +
>  		ret = virtnet_coal_params_supported(ec);
> +	}
>
>  	if (ret)
>  		return ret;
> --
> 2.17.1
>