Re: [PATCH netdev] virtio-net: support XDP_TX when not more queues

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jan 13, 2021 at 04:08:57PM +0800, Xuan Zhuo wrote:
> The number of queues implemented by many virtio backends is limited,
> especially some machines have a large number of CPUs. In this case, it
> is often impossible to allocate a separate queue for XDP_TX.
> 
> This patch allows XDP_TX to run by reuse the existing SQ with
> __netif_tx_lock() hold when there are not enough queues.
> 
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Reviewed-by: Dust Li <dust.li@linux.alibaba.com>

I'd like to get some advice on whether this is ok from some
XDP experts - previously my understanding was that it is
preferable to disable XDP for such devices than
use locks on XDP fast path.

> ---
>  drivers/net/virtio_net.c | 47 +++++++++++++++++++++++++++++++++++------------
>  1 file changed, 35 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index ba8e637..7a3b2a7 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -195,6 +195,9 @@ struct virtnet_info {
>  	/* # of XDP queue pairs currently used by the driver */
>  	u16 xdp_queue_pairs;
>  
> +	/* xdp_queue_pairs may be 0, when xdp is already loaded. So add this. */
> +	bool xdp_enabled;
> +
>  	/* I like... big packets and I cannot lie! */
>  	bool big_packets;
>  
> @@ -481,14 +484,34 @@ static int __virtnet_xdp_xmit_one(struct virtnet_info *vi,
>  	return 0;
>  }
>  
> -static struct send_queue *virtnet_xdp_sq(struct virtnet_info *vi)
> +static struct send_queue *virtnet_get_xdp_sq(struct virtnet_info *vi)
>  {
>  	unsigned int qp;
> +	struct netdev_queue *txq;
> +
> +	if (vi->curr_queue_pairs > nr_cpu_ids) {
> +		qp = vi->curr_queue_pairs - vi->xdp_queue_pairs + smp_processor_id();
> +	} else {
> +		qp = smp_processor_id() % vi->curr_queue_pairs;
> +		txq = netdev_get_tx_queue(vi->dev, qp);
> +		__netif_tx_lock(txq, raw_smp_processor_id());
> +	}
>  
> -	qp = vi->curr_queue_pairs - vi->xdp_queue_pairs + smp_processor_id();
>  	return &vi->sq[qp];
>  }
>  
> +static void virtnet_put_xdp_sq(struct virtnet_info *vi)
> +{
> +	unsigned int qp;
> +	struct netdev_queue *txq;
> +
> +	if (vi->curr_queue_pairs <= nr_cpu_ids) {
> +		qp = smp_processor_id() % vi->curr_queue_pairs;
> +		txq = netdev_get_tx_queue(vi->dev, qp);
> +		__netif_tx_unlock(txq);
> +	}
> +}
> +
>  static int virtnet_xdp_xmit(struct net_device *dev,
>  			    int n, struct xdp_frame **frames, u32 flags)
>  {
> @@ -512,7 +535,7 @@ static int virtnet_xdp_xmit(struct net_device *dev,
>  	if (!xdp_prog)
>  		return -ENXIO;
>  
> -	sq = virtnet_xdp_sq(vi);
> +	sq = virtnet_get_xdp_sq(vi);
>  
>  	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK)) {
>  		ret = -EINVAL;
> @@ -560,12 +583,13 @@ static int virtnet_xdp_xmit(struct net_device *dev,
>  	sq->stats.kicks += kicks;
>  	u64_stats_update_end(&sq->stats.syncp);
>  
> +	virtnet_put_xdp_sq(vi);
>  	return ret;
>  }
>  
>  static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
>  {
> -	return vi->xdp_queue_pairs ? VIRTIO_XDP_HEADROOM : 0;
> +	return vi->xdp_enabled ? VIRTIO_XDP_HEADROOM : 0;
>  }
>  
>  /* We copy the packet for XDP in the following cases:
> @@ -1457,12 +1481,13 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
>  		xdp_do_flush();
>  
>  	if (xdp_xmit & VIRTIO_XDP_TX) {
> -		sq = virtnet_xdp_sq(vi);
> +		sq = virtnet_get_xdp_sq(vi);
>  		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
>  			u64_stats_update_begin(&sq->stats.syncp);
>  			sq->stats.kicks++;
>  			u64_stats_update_end(&sq->stats.syncp);
>  		}
> +		virtnet_put_xdp_sq(vi);
>  	}
>  
>  	return received;
> @@ -2416,12 +2441,8 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>  		xdp_qp = nr_cpu_ids;
>  
>  	/* 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;
> -	}
> +	if (curr_qp + xdp_qp > vi->max_queue_pairs)
> +		xdp_qp = 0;
>  
>  	old_prog = rtnl_dereference(vi->rq[0].xdp_prog);
>  	if (!prog && !old_prog)
> @@ -2453,12 +2474,14 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>  	netif_set_real_num_rx_queues(dev, curr_qp + xdp_qp);
>  	vi->xdp_queue_pairs = xdp_qp;
>  
> +	vi->xdp_enabled = false;
>  	if (prog) {
>  		for (i = 0; i < vi->max_queue_pairs; i++) {
>  			rcu_assign_pointer(vi->rq[i].xdp_prog, prog);
>  			if (i == 0 && !old_prog)
>  				virtnet_clear_guest_offloads(vi);
>  		}
> +		vi->xdp_enabled = true;
>  	}
>  
>  	for (i = 0; i < vi->max_queue_pairs; i++) {
> @@ -2526,7 +2549,7 @@ static int virtnet_set_features(struct net_device *dev,
>  	int err;
>  
>  	if ((dev->features ^ features) & NETIF_F_LRO) {
> -		if (vi->xdp_queue_pairs)
> +		if (vi->xdp_enabled)
>  			return -EBUSY;
>  
>  		if (features & NETIF_F_LRO)
> -- 
> 1.8.3.1