[PATCH 0/3] virtio_net: introduce an RX queue watchdog for stuck detection

[PATCH 0/3] virtio_net: introduce an RX queue watchdog for stuck detection

[Longjun Tang]

From: Longjun Tang <tanglongjun@kylinos.cn>

Hi Michael, Kuba,

I once ran into a problem where the virtio_net receive queue became
permanently stuck.

The virtio_net RX path relies on backend interrupt notifications. 
The backend(vhost_net/vhost-user) appends buffers to the used ring
and then notifies the guest, relying on the hypervisor(KVM) to inject
the interrupt. On receiving it, virtio_net schedules NAPI, drains the
used ring and refills descriptors.

If the hypervisor fails to inject the interrupt(e.g. a transient KVM
failure), the guest never schedules NAPI, so it neither consumes buffers
nor returns descriptors. The backend keeps appending until the ring is full,
then stops and, with no free descriptors left, also stops sending
notifications. Both sides now wait for the other, and the RX queue is
permanently stuck.

The root cause is the lost interrupt in the hypervisor, and the proper
fix belongs there. Nevertheless, I believe virtio_net needs a way to
observe and diagnose it. Today, once a queue hangs, there is no signal
to the operator that anything is wrong: the RX path has no equivalent of
the TX watchdog (ndo_tx_timeout). This series lets virtio_net detect and
recover a stuck RX queue.


Implementation
--------------

Patch 1 exports virtqueue_get_last_used_idx(), a read-only accessor for
the last consumed used ring index. Together with the existing
virtqueue_poll(), it lets a driver ask whether the device has produced
buffers that the driver has not consumed yet ("backlog").

Patch 2 adds a per-device RX watchdog that scans all RX queues once per
second. A queue is considered stuck when, for rx_watchdog_timeo seconds
(default 5, 0 disables it), all of the following hold simultaneously:
  - the queue has a non-zero backlog (unconsumed buffers in the used ring);
  - the driver makes no consumption progress (last_used unchanged);
  - no new interrupt arrives (rq->calls unchanged).
On detection it logs a warning, rate-limited to once per timeout, and
reschedules NAPI, which drains the used ring and refills the descriptors,
so the queue recovers.

Patch 3 adds a per-queue rx_timeouts statistic, exposed via ethtool -S,
so the number of stuck events can be observed per queue.

questions and comments are welcome.

Thanks!
Lange

Longjun Tang (3):
  virtio: add virtqueue_get_last_used_idx() helper
  virtio_net: introduce an RX queue watchdog for stuck detection
  virtio_net: add rx_timeouts per-queue statistic

 drivers/net/virtio_net.c     | 112 +++++++++++++++++++++++++++++++++++
 drivers/virtio/virtio_ring.c |  18 +++++-
 include/linux/virtio.h       |   2 +
 3 files changed, 130 insertions(+), 2 deletions(-)

-- 
2.51.0

[PATCH 1/3] virtio: add virtqueue_get_last_used_idx() helper

[Longjun Tang]

From: Longjun Tang <tanglongjun@kylinos.cn>

Export a read-only accessor for the last consumed used ring index.
It's useful for drivers that need to detect whether the device has
produced buffers that have not been consumed yet, by passing the
returned value to virtqueue_poll().

The split ring now writes the field with WRITE_ONCE() (the packed
ring already did) to document that the new reader can run concurrently
with the poll path.

Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>
---
 drivers/virtio/virtio_ring.c | 18 ++++++++++++++++--
 include/linux/virtio.h       |  2 ++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index db678f5a80e0..6d8485c73881 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -1001,7 +1001,7 @@ static void *virtqueue_get_buf_ctx_split(struct vring_virtqueue *vq,
 	/* detach_buf_split clears data, so grab it now. */
 	ret = vq->split.desc_state[i].data;
 	detach_buf_split(vq, i, ctx);
-	vq->last_used_idx++;
+	WRITE_ONCE(vq->last_used_idx, vq->last_used_idx + 1);
 	/* If we expect an interrupt for the next entry, tell host
 	 * by writing event index and flush out the write before
 	 * the read in the next get_buf call. */
@@ -1068,7 +1068,7 @@ static void *virtqueue_get_buf_ctx_split_in_order(struct vring_virtqueue *vq,
 	ret = vq->split.desc_state[last_used].data;
 	detach_buf_split_in_order(vq, last_used, ctx);
 
-	vq->last_used_idx++;
+	WRITE_ONCE(vq->last_used_idx, vq->last_used_idx + 1);
 	vq->last_used += (vq->vq.num_free - num_free);
 	/* If we expect an interrupt for the next entry, tell host
 	 * by writing event index and flush out the write before
@@ -3227,6 +3227,20 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned int last_used_idx)
 }
 EXPORT_SYMBOL_GPL(virtqueue_poll);
 
+/**
+ * virtqueue_get_last_used_idx - get the last consumed used ring index
+ * @_vq: the struct virtqueue we're talking about.
+ *
+ * Returns the index of the most recently consumed used buffer.
+ */
+unsigned int virtqueue_get_last_used_idx(const struct virtqueue *_vq)
+{
+	const struct vring_virtqueue *vq = to_vvq(_vq);
+
+	return READ_ONCE(vq->last_used_idx);
+}
+EXPORT_SYMBOL_GPL(virtqueue_get_last_used_idx);
+
 /**
  * virtqueue_enable_cb - restart callbacks after disable_cb.
  * @_vq: the struct virtqueue we're talking about.
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index f923e42cfd01..1aed4d9235c8 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -112,6 +112,8 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
 
 bool virtqueue_poll(struct virtqueue *vq, unsigned);
 
+unsigned int virtqueue_get_last_used_idx(const struct virtqueue *vq);
+
 bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
 
 void *virtqueue_detach_unused_buf(struct virtqueue *vq);
-- 
2.51.0

Re: [PATCH 1/3] virtio: add virtqueue_get_last_used_idx() helper

[Jakub Kicinski]

On Mon,  9 Nov 2026 17:43:08 +0800 Longjun Tang wrote:
> +/**
> + * virtqueue_get_last_used_idx - get the last consumed used ring index
> + * @_vq: the struct virtqueue we're talking about.
> + *
> + * Returns the index of the most recently consumed used buffer.

kdoc needs Returns: (missing :)

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

[Longjun Tang]

From: Longjun Tang <tanglongjun@kylinos.cn>

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, reschedule NAPI.

Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>
---
 drivers/net/virtio_net.c | 104 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 104 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index dd2c41ef6fbe..ec7c248080c5 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
@@ -318,6 +322,13 @@ struct send_queue {
 	dma_addr_t xsk_hdr_dma_addr;
 };
 
+struct virtnet_rq_watchdog {
+	u16 last_used;
+	u16 calls;
+	unsigned long jiffies;
+	bool stalled;
+};
+
 /* Internal representation of a receive virtqueue */
 struct receive_queue {
 	/* Virtqueue associated with this receive_queue */
@@ -332,6 +343,9 @@ struct receive_queue {
 	/* The number of rx notifications */
 	u16 calls;
 
+	/* RX watchdog state. */
+	struct virtnet_rq_watchdog watchdog;
+
 	/* Is dynamic interrupt moderation enabled? */
 	bool dim_enabled;
 
@@ -443,6 +457,9 @@ struct virtnet_info {
 	/* Work struct for setting rx mode */
 	struct work_struct rx_mode_work;
 
+	/* RX watchdog timer for stuck detection */
+	struct timer_list rx_watchdog;
+
 	/* Does the affinity hint is set for virtqueues? */
 	bool affinity_hint_set;
 
@@ -3067,6 +3084,80 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
 	return received;
 }
 
+/*
+ * An RX queue is stalled if it has a backlog but makes no progress and
+ * gets no new interrupt for rx_watchdog_timeo seconds, usually because
+ * an interrupt was lost.  Mark it stalled and reschedule NAPI to drain
+ * the ring, refill the buffers.
+ */
+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);
+		bool progress = last_used != rq->watchdog.last_used;
+
+		if (!backlog || progress || calls != rq->watchdog.calls) {
+			rq->watchdog.last_used = 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, rescheduling NAPI\n",
+				    i, stall_ms);
+
+			rq->watchdog.stalled = true;
+			virtqueue_napi_schedule(&rq->napi, rq->vq);
+
+			/* Rate-limit to one event per timeout. */
+			rq->watchdog.jiffies = jiffies;
+		}
+	}
+
+	mod_timer(&vi->rx_watchdog, jiffies + HZ);
+}
+
+static void virtnet_rx_watchdog_init_queue(struct receive_queue *rq)
+{
+	rq->watchdog.last_used = virtqueue_get_last_used_idx(rq->vq);
+	rq->watchdog.calls = rq->calls;
+	rq->watchdog.jiffies = jiffies;
+	rq->watchdog.stalled = false;
+}
+
+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++)
+		virtnet_rx_watchdog_init_queue(&vi->rq[i]);
+
+	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]);
@@ -3230,6 +3321,8 @@ static int virtnet_open(struct net_device *dev)
 		netif_carrier_on(dev);
 	}
 
+	virtnet_rx_watchdog_start(vi);
+
 	return 0;
 
 err_enable_qp:
@@ -3759,6 +3852,7 @@ static int virtnet_set_queues(struct virtnet_info *vi, u16 queue_pairs)
 	struct virtio_net_rss_config_trailer old_rss_trailer;
 	struct net_device *dev = vi->dev;
 	struct scatterlist sg;
+	u16 old_queue_pairs = vi->curr_queue_pairs;
 
 	if (!vi->has_cvq || !virtio_has_feature(vi->vdev, VIRTIO_NET_F_MQ))
 		return 0;
@@ -3816,7 +3910,14 @@ static int virtnet_set_queues(struct virtnet_info *vi, u16 queue_pairs)
 	if (vi->has_rss)
 		vi->rss_trailer.max_tx_vq = cpu_to_le16(queue_pairs);
 succ:
+	if (dev->flags & IFF_UP) {
+		/* Init new queues */
+		for (int i = old_queue_pairs; i < queue_pairs; i++)
+			virtnet_rx_watchdog_init_queue(&vi->rq[i]);
+	}
+
 	vi->curr_queue_pairs = queue_pairs;
+
 	if (dev->flags & IFF_UP) {
 		local_bh_disable();
 		for (int i = 0; i < vi->curr_queue_pairs; ++i)
@@ -3832,6 +3933,8 @@ 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
 	 */
@@ -6900,6 +7003,7 @@ 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);
 	disable_work(&vi->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;
-- 
2.51.0

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

[Jakub Kicinski]

On Mon,  9 Nov 2026 17:43:09 +0800 Longjun Tang wrote:
> 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.

Can you say more about what platform you're hitting this problem on?
An what's the virtio backend?

[PATCH 3/3] virtio_net: add rx_timeouts per-queue statistic

[Longjun Tang]

From: Longjun Tang <tanglongjun@kylinos.cn>

Add an rx_timeouts field to the per-queue RX statistics and expose it
via ethtool. Whenever an RX queue gets stuck, the NAPI poll increments
rx_timeouts, so it counts RX queue stuck events.

Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>
---
 drivers/net/virtio_net.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index ec7c248080c5..feb6bfbb1c5a 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -135,6 +135,7 @@ struct virtnet_rq_stats {
 	u64_stats_t xdp_redirects;
 	u64_stats_t xdp_drops;
 	u64_stats_t kicks;
+	u64_stats_t rx_timeouts;
 };
 
 #define VIRTNET_SQ_STAT(name, m) {name, offsetof(struct virtnet_sq_stats, m), -1}
@@ -168,6 +169,7 @@ static const struct virtnet_stat_desc virtnet_rq_stats_desc[] = {
 	VIRTNET_RQ_STAT("xdp_redirects", xdp_redirects),
 	VIRTNET_RQ_STAT("xdp_drops",     xdp_drops),
 	VIRTNET_RQ_STAT("kicks",         kicks),
+	VIRTNET_RQ_STAT("rx_timeouts",   rx_timeouts),
 };
 
 static const struct virtnet_stat_desc virtnet_sq_stats_desc_qstat[] = {
@@ -2983,6 +2985,12 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
 		u64_stats_add(item, u64_stats_read(src));
 	}
 
+	/* Count a stall recovered by this poll. */
+	if (rq->watchdog.stalled) {
+		u64_stats_inc(&rq->stats.rx_timeouts);
+		rq->watchdog.stalled = false;
+	}
+
 	u64_stats_add(&rq->stats.packets, u64_stats_read(&stats.packets));
 	u64_stats_add(&rq->stats.bytes, u64_stats_read(&stats.bytes));
 	u64_stats_add(&rq->stats.alloc_fail, u64_stats_read(&stats.alloc_fail));
-- 
2.51.0