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 (`MAX_SKB_FRAGS` + 2).
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 (MAX_SKB_FRAGS + 2). 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@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/20250521092236.661410-4-lvivier@redhat.com
Tested-by: Lei Yang <leiyang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 031f8227531699c9dfaf7d128c4f1408764225b9..5d674eb9a0f2c60c0bf537b72ab6c736545c6a39 100644 (file)
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3504,6 +3504,12 @@ static int virtnet_tx_resize(struct virtnet_info *vi, struct send_queue *sq,
 {
        int qindex, err;
 
+       if (ring_num <= MAX_SKB_FRAGS + 2) {
+               netdev_err(vi->dev, "tx size (%d) cannot be smaller than %d\n",
+                          ring_num, MAX_SKB_FRAGS + 2);
+               return -EINVAL;
+       }
+
        qindex = sq - vi->sq;
 
        virtnet_tx_pause(vi, sq);