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

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

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
---
Changes in v2:
- Move the RCU locks inside the function (David)
- Use better helpers for waking up all the queues (Jakub)
- Link to v1: https://lore.kernel.org/r/20250701-netdev_flow_control-v1-1-240329fc91b1@debian.org
---
 drivers/net/netdevsim/bus.c    |  3 ++
 drivers/net/netdevsim/netdev.c | 63 ++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 64 insertions(+), 2 deletions(-)

diff --git a/drivers/net/netdevsim/bus.c b/drivers/net/netdevsim/bus.c
index 64c0cdd31bf85..1ba52471f3fbc 100644
--- a/drivers/net/netdevsim/bus.c
+++ b/drivers/net/netdevsim/bus.c
@@ -366,6 +366,9 @@ static ssize_t unlink_device_store(const struct bus_type *bus, const char *buf,
 	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);
diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index e36d3e846c2dc..b5b13fba6450d 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -37,9 +37,67 @@ MODULE_IMPORT_NS("NETDEV_INTERNAL");
 
 #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_peer_tx_queue(struct net_device *dev, struct nsim_rq *rq,
+				    u16 idx)
+{
+	struct netdevsim *ns = netdev_priv(dev);
+	struct net_device *peer_dev;
+	struct netdevsim *peer_ns;
+
+	rcu_read_lock();
+	peer_ns = rcu_dereference(ns->peer);
+	if (!peer_ns)
+		goto out;
+
+	/* TX device */
+	peer_dev = peer_ns->netdev;
+
+	/* If different queues size, do not stop, since it is not
+	 * easy to find which TX queue is mapped here
+	 */
+	if (dev->real_num_tx_queues != peer_dev->num_rx_queues)
+		goto out;
+
+	netif_subqueue_try_stop(peer_dev, idx,
+				NSIM_RING_SIZE - skb_queue_len(&rq->skb_queue),
+				NSIM_RING_SIZE / 2);
+out:
+	rcu_read_unlock();
+}
+
+static int nsim_napi_rx(struct net_device *dev, struct nsim_rq *rq,
+			struct sk_buff *skb)
 {
 	if (skb_queue_len(&rq->skb_queue) > NSIM_RING_SIZE) {
+		nsim_stop_peer_tx_queue(dev, rq, skb_get_queue_mapping(skb));
 		dev_kfree_skb_any(skb);
 		return NET_RX_DROP;
 	}
@@ -51,7 +109,7 @@ static int nsim_napi_rx(struct nsim_rq *rq, struct sk_buff *skb)
 static int nsim_forward_skb(struct net_device *dev, struct sk_buff *skb,
 			    struct nsim_rq *rq)
 {
-	return __dev_forward_skb(dev, skb) ?: nsim_napi_rx(rq, skb);
+	return __dev_forward_skb(dev, skb) ?: nsim_napi_rx(dev, rq, skb);
 }
 
 static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
@@ -351,6 +409,7 @@ static int nsim_rcv(struct nsim_rq *rq, int budget)
 			dev_dstats_rx_dropped(dev);
 	}
 
+	nsim_start_peer_tx_queue(dev, rq);
 	return i;
 }
 

---
base-commit: be4ea6c336b9a4fc1cc4be1c0549b24d0e687488
change-id: 20250630-netdev_flow_control-2b2d37965377

Best regards,
--  
Breno Leitao <leitao@debian.org>