netdev - Re: [PATCH RFC net-next] virtio_net: Relax queue requirement for using XDP

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Date:   Wed, 26 Feb 2020 11:00:40 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org,
        David Ahern <dahern@...italocean.com>,
        "Michael S . Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH RFC net-next] virtio_net: Relax queue requirement for
 using XDP


On 2020/2/26 上午8:57, David Ahern wrote:
> From: David Ahern <dahern@...italocean.com>
>
> virtio_net currently requires extra queues to install an XDP program,
> with the rule being twice as many queues as vcpus. From a host
> perspective this means the VM needs to have 2*vcpus vhost threads
> for each guest NIC for which XDP is to be allowed. For example, a
> 16 vcpu VM with 2 tap devices needs 64 vhost threads.
>
> The extra queues are only needed in case an XDP program wants to
> return XDP_TX. XDP_PASS, XDP_DROP and XDP_REDIRECT do not need
> additional queues. Relax the queue requirement and allow XDP
> functionality based on resources. If an XDP program is loaded and
> there are insufficient queues, then return a warning to the user
> and if a program returns XDP_TX just drop the packet. This allows
> the use of the rest of the XDP functionality to work without
> putting an unreasonable burden on the host.
>
> Cc: Jason Wang <jasowang@...hat.com>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Signed-off-by: David Ahern <dahern@...italocean.com>
> ---
>   drivers/net/virtio_net.c | 14 ++++++++++----
>   1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 2fe7a3188282..2f4c5b2e674d 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -190,6 +190,8 @@ struct virtnet_info {
>   	/* # of XDP queue pairs currently used by the driver */
>   	u16 xdp_queue_pairs;
>   
> +	bool can_do_xdp_tx;
> +
>   	/* I like... big packets and I cannot lie! */
>   	bool big_packets;
>   
> @@ -697,6 +699,8 @@ static struct sk_buff *receive_small(struct net_device *dev,
>   			len = xdp.data_end - xdp.data;
>   			break;
>   		case XDP_TX:
> +			if (!vi->can_do_xdp_tx)
> +				goto err_xdp;


I wonder if using spinlock to synchronize XDP_TX is better than dropping 
here?

Thanks


>   			stats->xdp_tx++;
>   			xdpf = convert_to_xdp_frame(&xdp);
>   			if (unlikely(!xdpf))
> @@ -870,6 +874,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   			}
>   			break;
>   		case XDP_TX:
> +			if (!vi->can_do_xdp_tx)
> +				goto err_xdp;
>   			stats->xdp_tx++;
>   			xdpf = convert_to_xdp_frame(&xdp);
>   			if (unlikely(!xdpf))
> @@ -2435,10 +2441,10 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>   
>   	/* XDP requires extra queues for XDP_TX */
>   	if (curr_qp + xdp_qp > vi->max_queue_pairs) {
> -		NL_SET_ERR_MSG_MOD(extack, "Too few free TX rings available");
> -		netdev_warn(dev, "request %i queues but max is %i\n",
> -			    curr_qp + xdp_qp, vi->max_queue_pairs);
> -		return -ENOMEM;
> +		NL_SET_ERR_MSG_MOD(extack, "Too few free TX rings available; XDP_TX will not be allowed");
> +		vi->can_do_xdp_tx = false;
> +	} else {
> +		vi->can_do_xdp_tx = true;
>   	}
>   
>   	old_prog = rtnl_dereference(vi->rq[0].xdp_prog);