[RFC PATCH 2/3] virtio_net: introduce an RX queue watchdog for stuck detection

From: Longjun Tang <[email protected]>

The backend raises an interrupt after adding buffers to the used ring,
and the hypervisor injects it. If one is lost (e.g., a transient KVM
failure), NAPI will not be scheduled and the driver stops consuming
buffers. The ring eventually fills up, the backend stops notifying, and
the queue is left permanently stuck.

Add a per-device watchdog that scans the RX queues once per second and
detects a queue that has a non-zero backlog while making no consumption
progress and receiving no new interrupt for rx_watchdog_timeo seconds
(default 5, 0 disables it). On detection it logs a warning, rate-limited
to once per timeout.

Assisted-by: Kilo:deepseek-v4-pro
Signed-off-by: Longjun Tang <[email protected]>
---
 drivers/net/virtio_net.c | 96 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 96 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index e34c52d059d3..02d0c4a67360 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -36,6 +36,10 @@ module_param(csum, bool, 0444);
 module_param(gso, bool, 0444);
 module_param(napi_tx, bool, 0644);
 
+/* RX queue stall timeout in seconds; 0 disables the watchdog. */
+static unsigned int rx_watchdog_timeo = 5;
+module_param(rx_watchdog_timeo, uint, 0644);
+
 #define VIRTIO_OFFLOAD_MAP_MIN	46
 #define VIRTIO_OFFLOAD_MAP_MAX	47
 #define VIRTIO_FEATURES_MAP_MIN	65
@@ -330,6 +334,11 @@ struct receive_queue {
 	/* The number of rx notifications */
 	u16 calls;
 
+	/* RX watchdog state for lost-interrupt detection. */
+	u16 watchdog_last_used_idx;
+	u16 watchdog_calls;
+	unsigned long watchdog_jiffies;
+
 	/* Is dynamic interrupt moderation enabled? */
 	bool dim_enabled;
 
@@ -441,6 +450,9 @@ struct virtnet_info {
 	/* Work struct for setting rx mode */
 	struct work_struct rx_mode_work;
 
+	/* RX watchdog timer for lost-interrupt detection */
+	struct timer_list rx_watchdog;
+
 	/* OK to queue work setting RX mode? */
 	bool rx_mode_work_enabled;
 
@@ -3046,6 +3058,85 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
 	return received;
 }
 
+/*
+ * The RX path is entirely event driven: the driver relies on the
+ * hypervisor injecting an interrupt for every used buffer. If an
+ * interrupt is lost (e.g. a transient KVM failure), NAPI is never
+ * scheduled, the driver stops consuming buffers, the backend fills the
+ * ring and, once it runs out of descriptors, stops sending further
+ * notifications. Both sides then wait for the other and the queue is
+ * permanently stuck.
+ *
+ * This watchdog detects that state: a queue is considered stalled when
+ * it has a non-zero backlog, makes no consumption progress and receives
+ * no new interrupt for rx_watchdog_timeo seconds. On detection it logs
+ * a warning.
+ */
+static void virtnet_rx_watchdog(struct timer_list *t)
+{
+	struct virtnet_info *vi = timer_container_of(vi, t, rx_watchdog);
+	unsigned long timeout = rx_watchdog_timeo * HZ;
+	int i;
+
+	if (!rx_watchdog_timeo)
+		return;
+
+	for (i = 0; i < vi->curr_queue_pairs; i++) {
+		struct receive_queue *rq = &vi->rq[i];
+		u16 last_used = virtqueue_get_last_used_idx(rq->vq);
+		u16 calls = rq->calls;
+		bool backlog = virtqueue_poll(rq->vq, last_used);
+
+		if (!backlog || last_used != rq->watchdog_last_used_idx ||
+		    calls != rq->watchdog_calls) {
+			/* No pending data, or the queue made progress, or a
+			 * new interrupt arrived: restart the window.
+			 */
+			rq->watchdog_last_used_idx = last_used;
+			rq->watchdog_calls = calls;
+			rq->watchdog_jiffies = jiffies;
+			continue;
+		}
+
+		if (time_after(jiffies, rq->watchdog_jiffies + timeout)) {
+			unsigned int stall_ms =
+				jiffies_to_msecs(jiffies - rq->watchdog_jiffies);
+
+			netdev_warn(vi->dev, "RX queue %u stalled for %u ms\n",
+				    i, stall_ms);
+
+			/* Rate-limit to one event per timeout. */
+			rq->watchdog_jiffies = jiffies;
+		}
+	}
+
+	mod_timer(&vi->rx_watchdog, jiffies + HZ);
+}
+
+static void virtnet_rx_watchdog_start(struct virtnet_info *vi)
+{
+	int i;
+
+	if (!rx_watchdog_timeo)
+		return;
+
+	for (i = 0; i < vi->curr_queue_pairs; i++) {
+		struct receive_queue *rq = &vi->rq[i];
+
+		rq->watchdog_last_used_idx =
+			virtqueue_get_last_used_idx(rq->vq);
+		rq->watchdog_calls = rq->calls;
+		rq->watchdog_jiffies = jiffies;
+	}
+
+	mod_timer(&vi->rx_watchdog, jiffies + HZ);
+}
+
+static void virtnet_rx_watchdog_stop(struct virtnet_info *vi)
+{
+	timer_delete_sync(&vi->rx_watchdog);
+}
+
 static void virtnet_disable_queue_pair(struct virtnet_info *vi, int qp_index)
 {
 	virtnet_napi_tx_disable(&vi->sq[qp_index]);
@@ -3209,6 +3300,8 @@ static int virtnet_open(struct net_device *dev)
 		netif_carrier_on(dev);
 	}
 
+	virtnet_rx_watchdog_start(vi);
+
 	return 0;
 
 err_enable_qp:
@@ -3802,6 +3895,8 @@ static int virtnet_close(struct net_device *dev)
 	struct virtnet_info *vi = netdev_priv(dev);
 	int i;
 
+	virtnet_rx_watchdog_stop(vi);
+
 	/* Prevent the config change callback from changing carrier
 	 * after close
 	 */
@@ -6869,6 +6964,7 @@ static int virtnet_probe(struct virtio_device *vdev)
 
 	INIT_WORK(&vi->config_work, virtnet_config_changed_work);
 	INIT_WORK(&vi->rx_mode_work, virtnet_rx_mode_work);
+	timer_setup(&vi->rx_watchdog, virtnet_rx_watchdog, 0);
 
 	if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF)) {
 		vi->mergeable_rx_bufs = true;
-- 
2.43.0