[PATCH v6 net-next] virtio-net: support XDP when not more queues

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

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/XDP_REDIRECT, then xdp cannot be loaded to work, even xdp does
not use the XDP_TX/XDP_REDIRECT.

This patch allows XDP_TX/XDP_REDIRECT 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>
---
v6: 1. use __netif_tx_acquire()/__netif_tx_release(). (suggested by Jason Wang)
    2. add note for why not lock. (suggested by Jason Wang)
    3. Use variable 'flag' to record with or without locked.  It is not safe to
       use curr_queue_pairs in "virtnet_put_xdp_sq", because it may changed after
       "virtnet_get_xdp_sq".

v5: change subject from 'support XDP_TX when not more queues'

v4: make sparse happy
    suggested by Jakub Kicinski

v3: add warning when no more queues
    suggested by Jesper Dangaard Brouer

 drivers/net/virtio_net.c | 63 ++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 53 insertions(+), 10 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index ba8e637..f9e024d 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 +484,48 @@ 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, int *flag)
+	__acquires(txq->_xmit_lock)
 {
+	struct netdev_queue *txq;
 	unsigned int qp;

-	qp = vi->curr_queue_pairs - vi->xdp_queue_pairs + smp_processor_id();
+	if (vi->curr_queue_pairs > nr_cpu_ids) {
+		qp = vi->curr_queue_pairs - vi->xdp_queue_pairs + smp_processor_id();
+		txq = netdev_get_tx_queue(vi->dev, qp);
+
+		/* In this case, this txq is only used for xdp tx on the current
+		 * cpu, so it does not need to be locked.
+		 * __netif_tx_acquire is for sparse.
+		 */
+		__netif_tx_acquire(txq);
+		*flag = false;
+	} 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());
+		*flag = true;
+	}
+
 	return &vi->sq[qp];
 }

+static void virtnet_put_xdp_sq(struct virtnet_info *vi, struct send_queue *sq,
+			       int flag)
+	__releases(txq->_xmit_lock)
+{
+	struct netdev_queue *txq;
+	unsigned int qp;
+
+	qp = sq - vi->sq;
+	txq = netdev_get_tx_queue(vi->dev, qp);
+
+	if (flag)
+		__netif_tx_unlock(txq);
+	else
+		__netif_tx_release(txq);
+}
+
 static int virtnet_xdp_xmit(struct net_device *dev,
 			    int n, struct xdp_frame **frames, u32 flags)
 {
@@ -496,12 +533,12 @@ static int virtnet_xdp_xmit(struct net_device *dev,
 	struct receive_queue *rq = vi->rq;
 	struct bpf_prog *xdp_prog;
 	struct send_queue *sq;
+	int ret, err, sq_flag;
 	unsigned int len;
 	int packets = 0;
 	int bytes = 0;
 	int drops = 0;
 	int kicks = 0;
-	int ret, err;
 	void *ptr;
 	int i;

@@ -512,7 +549,7 @@ 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, &sq_flag);

 	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK)) {
 		ret = -EINVAL;
@@ -560,12 +597,13 @@ 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, sq, sq_flag);
 	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 +1495,15 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
 		xdp_do_flush();

 	if (xdp_xmit & VIRTIO_XDP_TX) {
-		sq = virtnet_xdp_sq(vi);
+		int sq_flag;
+
+		sq = virtnet_get_xdp_sq(vi, &sq_flag);
 		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, sq, sq_flag);
 	}

 	return received;
@@ -2417,10 +2458,9 @@ 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",
+		netdev_warn(dev, "XDP request %i queues but max is %i. XDP_TX and XDP_REDIRECT will operate in a slower locked tx mode.\n",
 			    curr_qp + xdp_qp, vi->max_queue_pairs);
-		return -ENOMEM;
+		xdp_qp = 0;
 	}

 	old_prog = rtnl_dereference(vi->rq[0].xdp_prog);
@@ -2454,11 +2494,14 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
 	vi->xdp_queue_pairs = xdp_qp;

 	if (prog) {
+		vi->xdp_enabled = true;
 		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);
 		}
+	} else {
+		vi->xdp_enabled = false;
 	}

 	for (i = 0; i < vi->max_queue_pairs; i++) {
@@ -2526,7 +2569,7 @@ 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