netdev - [PATCH 2/2] virtio_net: Enforce minimum TX ring size for reliability

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Message-ID: <20250520110526.635507-3-lvivier@redhat.com>
Date: Tue, 20 May 2025 13:05:26 +0200
From: Laurent Vivier <lvivier@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: Jason Wang <jasowang@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	netdev@...r.kernel.org,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: [PATCH 2/2] virtio_net: Enforce minimum TX ring size for reliability

The `tx_may_stop()` logic stops TX queues if free descriptors
(`sq->vq->num_free`) fall below the threshold of (2 + `MAX_SKB_FRAGS`).
If the total ring size (`ring_num`) is not strictly greater than this
value, queues can become persistently stopped or stop after minimal
use, severely degrading performance.

A single sk_buff transmission typically requires descriptors for:
- The virtio_net_hdr (1 descriptor)
- The sk_buff's linear data (head) (1 descriptor)
- Paged fragments (up to MAX_SKB_FRAGS descriptors)

This patch enforces that the TX ring size ('ring_num') must be strictly
greater than (2 + MAX_SKB_FRAGS). This ensures that the ring is
always large enough to hold at least one maximally-fragmented packet
plus at least one additional slot.

Reported-by: Lei Yang <leiyang@...hat.com>
Signed-off-by: Laurent Vivier <lvivier@...hat.com>
---
 drivers/net/virtio_net.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index e53ba600605a..866961f368a2 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3481,6 +3481,12 @@ static int virtnet_tx_resize(struct virtnet_info *vi, struct send_queue *sq,
 {
 	int qindex, err;
 
+	if (ring_num <= 2+MAX_SKB_FRAGS) {
+		netdev_err(vi->dev, "tx size (%d) cannot be smaller than %d\n",
+			   ring_num, 2+MAX_SKB_FRAGS);
+		return -EINVAL;
+	}
+
 	qindex = sq - vi->sq;
 
 	virtnet_tx_pause(vi, sq);
-- 
2.49.0