[PATCH net] veth: fix queue index used to wake the peer txq in veth_poll

["Jonas Köppeler" <j.koeppeler@tu-berlin.de>]

veth_poll() derives the index of the peer TX queue to wake from
rq->xdp_rxq.queue_index. That field is only initialized by
xdp_rxq_info_reg() in veth_enable_xdp_range(), which runs only when an
XDP program is attached. On the plain GRO/NAPI path
(veth_napi_enable_range()) xdp_rxq_info_reg() is never called, so
queue_index stays 0 for every queue, as priv->rq is zero-allocated. The
same happens once an XDP program is detached while GRO stays enabled,
since xdp_rxq_info_unreg() clears the struct again.

So in a multi-queue setup with GRO enabled and no XDP program attached,
every NAPI instance looks at the peer's TX queue 0. If veth_xmit() stops
peer TX queue 1 because the ptr_ring is full (NETDEV_TX_BUSY), nothing
ever wakes it again: the poller draining queue 1 wakes queue 0 instead.
veth implements no ndo_tx_timeout, so the netdev watchdog does not kick
in either, and the queue stays stopped indefinitely.

Derive the index from the position of the rq within priv->rq instead,
which is correct regardless of whether XDP was ever enabled.

Scripts to reproduce the stall are available at
https://github.com/netoptimizer/veth-backpressure-performance-testing

Fixes: dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops")
Signed-off-by: Jonas Köppeler <j.koeppeler@tu-berlin.de>
Tested-by: Jesper Dangaard Brouer <hawk@kernel.org>
---
 drivers/net/veth.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 00e34afd858e..e927ef3f47b3 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -961,7 +961,7 @@ static int veth_poll(struct napi_struct *napi, int budget)
 	struct veth_rq *rq =
 		container_of(napi, struct veth_rq, xdp_napi);
 	struct veth_priv *priv = netdev_priv(rq->dev);
-	int queue_idx = rq->xdp_rxq.queue_index;
+	int queue_idx = rq - priv->rq;
 	struct netdev_queue *peer_txq;
 	struct veth_stats stats = {};
 	struct net_device *peer_dev;

---
base-commit: 44871eadd07a7f004aa00cb87399461eea08c630
change-id: 20260805-veth-fix-poll-queue-idx-9d47909b033d

Best regards,
--  
Jonas Köppeler <j.koeppeler@tu-berlin.de>