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 <maciej.fijalkowski@intel.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: Magnus Karlsson <magnus.karlsson@intel.com> Link: https://lore.kernel.org/bpf/20220707130842.49408-1-maciej.fijalkowski@intel.com

Author: mfijalko

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;