[PATCH net V2 2/2] vhost-net: correctly flush batched packet before enabling notification

[Jason Wang <jasowang@redhat.com>]

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.

Fixing this via partially reverting 8c2e6b26ffe2 and sticking the
notification enabling logic inside the loop when nothing new is
spotted and flush the batched before.

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>
---
Changes since V1:
- Tweak the commit log
- Typo fixes
---
 drivers/vhost/net.c | 33 +++++++++++++--------------------
 1 file changed, 13 insertions(+), 20 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 16e39f3ab956..3611b7537932 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 +780,18 @@ 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.
+			/* Flush batched packets before enabling
+			 * virqtueue notification to reduce
+			 * unnecssary virtqueue kicks.
 			 */
+			vhost_tx_batch(net, nvq, sock, &msg);
+			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 +847,7 @@ 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)
-- 
2.34.1