Commit 5ef44b3cb43b ("xsk: Bring back busy polling support") fixed the
busy polling support in xsk for XDP_ZEROCOPY after it was broken in
commit 86e25f40aa1e ("net: napi: Add napi_config"). The busy polling
support with XDP_COPY remained broken since the napi_id setup in
xsk_rcv_check was removed.

Bring back the setup of napi_id for XDP_COPY so socket level SO_BUSYPOLL
can be used to poll the underlying napi.

Tested using AF_XDP support in virtio-net by running the xsk_rr AF_XDP
benchmarking tool shared here:
https://lore.kernel.org/all/20250320163523.3501305-1-skhawaja@google.com/T/ (local)

Enabled socket busy polling using following commands in qemu,

sudo ethtool -L eth0 combined 1
sudo ethtool -G eth0 rx 1024
echo 400 | sudo tee /proc/sys/net/core/busy_read
echo 100 | sudo tee /sys/class/net/eth0/napi_defer_hard_irqs
echo 15000   | sudo tee /sys/class/net/eth0/gro_flush_timeout

Fixes: 5ef44b3cb43b ("xsk: Bring back busy polling support")
Fixes: 86e25f40aa1e ("net: napi: Add napi_config")
Signed-off-by: Samiullah Khawaja <redacted>
---
 net/xdp/xsk.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index e5d104ce7b82..de8bf97b2cb9 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c

@@ -310,6 +310,18 @@ static bool xsk_is_bound(struct xdp_sock *xs)
        return false;
 }

+static void __xsk_mark_napi_id_once(struct sock *sk, struct net_device *dev, u32 qid)
+{
+       struct netdev_rx_queue *rxq;
+
+       if (qid >= dev->real_num_rx_queues)
+               return;
+
+       rxq = __netif_get_rx_queue(dev, qid);
+       if (rxq->napi)
+               __sk_mark_napi_id_once(sk, rxq->napi->napi_id);
+}
+
 static int xsk_rcv_check(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
 {
        if (!xsk_is_bound(xs))

@@ -323,6 +335,7 @@ static int xsk_rcv_check(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
                return -ENOSPC;
        }

+       __xsk_mark_napi_id_once(&xs->sk, xs->dev, xs->queue_id);
        return 0;
 }

@@ -1300,13 +1313,8 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
        xs->queue_id = qid;
        xp_add_xsk(xs->pool, xs);

-       if (xs->zc && qid < dev->real_num_rx_queues) {
-               struct netdev_rx_queue *rxq;
-
-               rxq = __netif_get_rx_queue(dev, qid);
-               if (rxq->napi)
-                       __sk_mark_napi_id_once(sk, rxq->napi->napi_id);
-       }
+       if (xs->zc)
+               __xsk_mark_napi_id_once(sk, dev, qid);

 out_unlock:
        if (err) {
--

2.49.0.395.g12beb8f557-goog