netdevsim: implement peer queue flow control

Add flow control mechanism between paired netdevsim devices to stop the
TX queue during high traffic scenarios. When a receive queue becomes
congested (approaching NSIM_RING_SIZE limit), the corresponding transmit
queue on the peer device is stopped using netif_subqueue_try_stop().

Once the receive queue has sufficient capacity again, the peer's
transmit queue is resumed with netif_tx_wake_queue().

Key changes:
* Add nsim_stop_peer_tx_queue() to pause peer TX when RX queue is full
* Add nsim_start_peer_tx_queue() to resume peer TX when RX queue drains
* Implement queue mapping validation to ensure TX/RX queue count match
* Wake all queues during device unlinking to prevent stuck queues
* Use RCU protection when accessing peer device references
* wake the queues when changing the queue numbers
* Remove IFF_NO_QUEUE given it will enqueue packets now

The flow control only activates when devices have matching TX/RX queue
counts to ensure proper queue mapping.

Suggested-by: Jakub Kicinski <[e-mail]>
Signed-off-by: Breno Leitao <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/netdevsim/bus.c

···
 	err = 0;
 	RCU_INIT_POINTER(nsim->peer, NULL);
 	RCU_INIT_POINTER(peer->peer, NULL);
+	synchronize_net();
+	netif_tx_wake_all_queues(dev);
+	netif_tx_wake_all_queues(peer->netdev);
 
 out_put_netns:
 	put_net(ns);

drivers/net/netdevsim/ethtool.c

···
 	ch->combined_count = ns->ethtool.channels;
 }
 
+static void
+nsim_wake_queues(struct net_device *dev)
+{
+	struct netdevsim *ns = netdev_priv(dev);
+	struct netdevsim *peer;
+
+	synchronize_net();
+	netif_tx_wake_all_queues(dev);
+
+	rcu_read_lock();
+	peer = rcu_dereference(ns->peer);
+	if (peer)
+		netif_tx_wake_all_queues(peer->netdev);
+	rcu_read_unlock();
+}
+
 static int
 nsim_set_channels(struct net_device *dev, struct ethtool_channels *ch)
 {
···
 		return err;
 
 	ns->ethtool.channels = ch->combined_count;
+
+	/* Only wake up queues if devices are linked */
+	if (rcu_access_pointer(ns->peer))
+		nsim_wake_queues(dev);
+
 	return 0;
 }
 

drivers/net/netdevsim/netdev.c

···
 
 #define NSIM_RING_SIZE		256
 
-static int nsim_napi_rx(struct nsim_rq *rq, struct sk_buff *skb)
+static void nsim_start_peer_tx_queue(struct net_device *dev, struct nsim_rq *rq)
+{
+	struct netdevsim *ns = netdev_priv(dev);
+	struct net_device *peer_dev;
+	struct netdevsim *peer_ns;
+	struct netdev_queue *txq;
+	u16 idx;
+
+	idx = rq->napi.index;
+	rcu_read_lock();
+	peer_ns = rcu_dereference(ns->peer);
+	if (!peer_ns)
+		goto out;
+
+	/* TX device */
+	peer_dev = peer_ns->netdev;
+	if (dev->real_num_tx_queues != peer_dev->num_rx_queues)
+		goto out;
+
+	txq = netdev_get_tx_queue(peer_dev, idx);
+	if (!netif_tx_queue_stopped(txq))
+		goto out;
+
+	netif_tx_wake_queue(txq);
+out:
+	rcu_read_unlock();
+}
+
+static void nsim_stop_tx_queue(struct net_device *tx_dev,
+			       struct net_device *rx_dev,
+			       struct nsim_rq *rq,
+			       u16 idx)
+{
+	/* If different queues size, do not stop, since it is not
+	 * easy to find which TX queue is mapped here
+	 */
+	if (rx_dev->real_num_tx_queues != tx_dev->num_rx_queues)
+		return;
+
+	/* rq is the queue on the receive side */
+	netif_subqueue_try_stop(tx_dev, idx,
+				NSIM_RING_SIZE - skb_queue_len(&rq->skb_queue),
+				NSIM_RING_SIZE / 2);
+}
+
+static int nsim_napi_rx(struct net_device *tx_dev, struct net_device *rx_dev,
+			struct nsim_rq *rq, struct sk_buff *skb)
 {
 	if (skb_queue_len(&rq->skb_queue) > NSIM_RING_SIZE) {
 		dev_kfree_skb_any(skb);
···
 	}
 
 	skb_queue_tail(&rq->skb_queue, skb);
+
+	/* Stop the peer TX queue avoiding dropping packets later */
+	if (skb_queue_len(&rq->skb_queue) >= NSIM_RING_SIZE)
+		nsim_stop_tx_queue(tx_dev, rx_dev, rq,
+				   skb_get_queue_mapping(skb));
+
 	return NET_RX_SUCCESS;
 }
 
-static int nsim_forward_skb(struct net_device *dev, struct sk_buff *skb,
+static int nsim_forward_skb(struct net_device *tx_dev,
+			    struct net_device *rx_dev,
+			    struct sk_buff *skb,
 			    struct nsim_rq *rq)
 {
-	return __dev_forward_skb(dev, skb) ?: nsim_napi_rx(rq, skb);
+	return __dev_forward_skb(rx_dev, skb) ?:
+		nsim_napi_rx(tx_dev, rx_dev, rq, skb);
 }
 
 static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
···
 		skb_linearize(skb);
 
 	skb_tx_timestamp(skb);
-	if (unlikely(nsim_forward_skb(peer_dev, skb, rq) == NET_RX_DROP))
+	if (unlikely(nsim_forward_skb(dev, peer_dev, skb, rq) == NET_RX_DROP))
 		goto out_drop_cnt;
 
 	if (!hrtimer_active(&rq->napi_timer))
···
 			dev_dstats_rx_dropped(dev);
 	}
 
+	nsim_start_peer_tx_queue(dev, rq);
 	return i;
 }
 
···
 	ether_setup(dev);
 	eth_hw_addr_random(dev);
 
-	dev->tx_queue_len = 0;
 	dev->flags &= ~IFF_MULTICAST;
-	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE |
-			   IFF_NO_QUEUE;
+	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE;
 	dev->features |= NETIF_F_HIGHDMA |
 			 NETIF_F_SG |
 			 NETIF_F_FRAGLIST |