xsk: Mark napi_id on sendmsg()

When application runs in busy poll mode and does not receive a single
packet but only sends them, it is currently impossible to get into
napi_busy_loop() as napi_id is only marked on Rx side in xsk_rcv_check().
In there, napi_id is being taken from xdp_rxq_info carried by xdp_buff.
From Tx perspective, we do not have access to it. What we have handy is
the xsk pool.

Xsk pool works on a pool of internal xdp_buff wrappers called xdp_buff_xsk.
AF_XDP ZC enabled drivers call xp_set_rxq_info() so each of xdp_buff_xsk
has a valid pointer to xdp_rxq_info of underlying queue. Therefore, on Tx
side, napi_id can be pulled from xs->pool->heads[0].xdp.rxq->napi_id. Hide
this pointer chase under helper function, xsk_pool_get_napi_id().

Do this only for sockets working in ZC mode as otherwise rxq pointers would
not be initialized.

Signed-off-by: Maciej Fijalkowski <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Magnus Karlsson <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

include/net/xdp_sock_drv.h

@@ -44,6 +44,15 @@ static inline void xsk_pool_set_rxq_info(struct xsk_buff_pool *pool,
 	xp_set_rxq_info(pool, rxq);
 }
 
+static inline unsigned int xsk_pool_get_napi_id(struct xsk_buff_pool *pool)
+{
+#ifdef CONFIG_NET_RX_BUSY_POLL
+	return pool->heads[0].xdp.rxq->napi_id;
+#else
+	return 0;
+#endif
+}
+
 static inline void xsk_pool_dma_unmap(struct xsk_buff_pool *pool,
 				      unsigned long attrs)
 {
@@ -198,6 +207,11 @@ static inline void xsk_pool_set_rxq_info(struct xsk_buff_pool *pool,
 {
 }
 
+static inline unsigned int xsk_pool_get_napi_id(struct xsk_buff_pool *pool)
+{
+	return 0;
+}
+
 static inline void xsk_pool_dma_unmap(struct xsk_buff_pool *pool,
 				      unsigned long attrs)
 {

net/xdp/xsk.c

@@ -639,8 +639,11 @@ static int __xsk_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len
 	if (unlikely(need_wait))
 		return -EOPNOTSUPP;
 
-	if (sk_can_busy_loop(sk))
+	if (sk_can_busy_loop(sk)) {
+		if (xs->zc)
+			__sk_mark_napi_id_once(sk, xsk_pool_get_napi_id(xs->pool));
 		sk_busy_loop(sk, 1); /* only support non-blocking sockets */
+	}
 
 	if (xs->zc && xsk_no_wakeup(sk))
 		return 0;