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

[Jason Wang <jasowang@xxxxxxxxxx>]

On 2021/2/11 5:40 上午, Michael S. Tsirkin wrote:
> 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@xxxxxxxxxxxxxxxxx>
> > Reviewed-by: Dust Li <dust.li@xxxxxxxxxxxxxxxxx>
> 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.


I think this is acceptable on the device that changing the number of 
queues is not easy. For virtio-net, it probably requires a lot of 
changes in the management.

Another example is TUN which use TX lock for XDP.

Thanks


> > ---
> >   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