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;
 	}