[PATCH v3 1/3] vhost-net: unbreak busy polling

[PATCH v3 1/3] vhost-net: unbreak busy polling

[Michael S. Tsirkin]

From: Jason Wang <jasowang@redhat.com>

Commit 67a873df0c41 ("vhost: basic in order support") pass the number
of used elem to vhost_net_rx_peek_head_len() to make sure it can
signal the used correctly before trying to do busy polling. But it
forgets to clear the count, this would cause the count run out of sync
with handle_rx() and break the busy polling.

Fixing this by passing the pointer of the count and clearing it after
the signaling the used.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 67a873df0c41 ("vhost: basic in order support")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20250915024703.2206-1-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/vhost/net.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index c6508fe0d5c8..16e39f3ab956 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -1014,7 +1014,7 @@ static int peek_head_len(struct vhost_net_virtqueue *rvq, struct sock *sk)
 }
 
 static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk,
-				      bool *busyloop_intr, unsigned int count)
+				      bool *busyloop_intr, unsigned int *count)
 {
 	struct vhost_net_virtqueue *rnvq = &net->vqs[VHOST_NET_VQ_RX];
 	struct vhost_net_virtqueue *tnvq = &net->vqs[VHOST_NET_VQ_TX];
@@ -1024,7 +1024,8 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk,
 
 	if (!len && rvq->busyloop_timeout) {
 		/* Flush batched heads first */
-		vhost_net_signal_used(rnvq, count);
+		vhost_net_signal_used(rnvq, *count);
+		*count = 0;
 		/* Both tx vq and rx socket were polled here */
 		vhost_net_busy_poll(net, rvq, tvq, busyloop_intr, true);
 
@@ -1180,7 +1181,7 @@ static void handle_rx(struct vhost_net *net)
 
 	do {
 		sock_len = vhost_net_rx_peek_head_len(net, sock->sk,
-						      &busyloop_intr, count);
+						      &busyloop_intr, &count);
 		if (!sock_len)
 			break;
 		sock_len += sock_hlen;
-- 
MST

[PATCH v3 2/3] Revert "vhost/net: Defer TX queue re-enable until after sendmsg"

[Michael S. Tsirkin]

Commit 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after
sendmsg") tries to defer the notification enabling by moving the logic
out of the loop after the vhost_tx_batch() when nothing new is
spotted. This will bring side effects as the new logic would be reused
for several other error conditions.

One example is the IOTLB: when there's an IOTLB miss, get_tx_bufs()
might return -EAGAIN and exit the loop and see there's still available
buffers, so it will queue the tx work again until userspace feed the
IOTLB entry correctly. This will slowdown the tx processing and
trigger the TX watchdog in the guest as reported in
https://lkml.org/lkml/2025/9/10/1596.

To fix, revert the change. A follow up patch will being the performance
back in a safe way.

Link: https://lkml.org/lkml/2025/9/10/1596.
Reported-by: Jon Kohler <jon@nutanix.com>
Cc: stable@vger.kernel.org
Fixes: 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after sendmsg")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/vhost/net.c | 30 +++++++++---------------------
 1 file changed, 9 insertions(+), 21 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 16e39f3ab956..57efd5c55f89 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -765,11 +765,11 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
 	int err;
 	int sent_pkts = 0;
 	bool sock_can_batch = (sock->sk->sk_sndbuf == INT_MAX);
-	bool busyloop_intr;
 	bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
 
 	do {
-		busyloop_intr = false;
+		bool busyloop_intr = false;
+
 		if (nvq->done_idx == VHOST_NET_BATCH)
 			vhost_tx_batch(net, nvq, sock, &msg);
 
@@ -780,10 +780,13 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
 			break;
 		/* Nothing new?  Wait for eventfd to tell us they refilled. */
 		if (head == vq->num) {
-			/* Kicks are disabled at this point, break loop and
-			 * process any remaining batched packets. Queue will
-			 * be re-enabled afterwards.
-			 */
+			if (unlikely(busyloop_intr)) {
+				vhost_poll_queue(&vq->poll);
+			} else if (unlikely(vhost_enable_notify(&net->dev,
+								vq))) {
+				vhost_disable_notify(&net->dev, vq);
+				continue;
+			}
 			break;
 		}
 
@@ -839,22 +842,7 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
 		++nvq->done_idx;
 	} while (likely(!vhost_exceeds_weight(vq, ++sent_pkts, total_len)));
 
-	/* Kicks are still disabled, dispatch any remaining batched msgs. */
 	vhost_tx_batch(net, nvq, sock, &msg);
-
-	if (unlikely(busyloop_intr))
-		/* If interrupted while doing busy polling, requeue the
-		 * handler to be fair handle_rx as well as other tasks
-		 * waiting on cpu.
-		 */
-		vhost_poll_queue(&vq->poll);
-	else
-		/* All of our work has been completed; however, before
-		 * leaving the TX handler, do one last check for work,
-		 * and requeue handler if necessary. If there is no work,
-		 * queue will be reenabled.
-		 */
-		vhost_net_busy_poll_try_queue(net, vq);
 }
 
 static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
-- 
MST

[PATCH v3 3/3] vhost-net: flush batched before enabling notifications

[Michael S. Tsirkin]

Commit 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after
sendmsg") tries to defer the notification enabling by moving the logic
out of the loop after the vhost_tx_batch() when nothing new is spotted.
This caused unexpected side effects as the new logic is reused for
several other error conditions.

A previous patch reverted 8c2e6b26ffe2. Now, bring the performance
back up by flushing batched buffers before enabling notifications.

Link: https://lore.kernel.org/all/20250915024703.2206-2-jasowang@redhat.com
Reported-by: Jon Kohler <jon@nutanix.com>
Cc: stable@vger.kernel.org
Fixes: 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after sendmsg")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/vhost/net.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 57efd5c55f89..72ecb8691275 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -782,11 +782,18 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
 		if (head == vq->num) {
 			if (unlikely(busyloop_intr)) {
 				vhost_poll_queue(&vq->poll);
-			} else if (unlikely(vhost_enable_notify(&net->dev,
-								vq))) {
-				vhost_disable_notify(&net->dev, vq);
-				continue;
-			}
+			} else {
+				/* Flush batched packets before enabling
+				 * virtqueue notifications to reduce
+				 * unnecessary virtqueue kicks.
+				 */
+				vhost_tx_batch(net, nvq, sock, &msg);
+
+				if (unlikely(vhost_enable_notify(&net->dev,
+								 vq))) {
+					vhost_disable_notify(&net->dev, vq);
+					continue;
+				}
 			break;
 		}
 
-- 
MST