[PATCH v2 bpf-next] xsk: mark napi_id on sendmsg()

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

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>
---

v2:
* target bpf-next instead of bpf and don't treat it as fix (Bjorn)
* hide pointer chasing under helper function (Bjorn)

 include/net/xdp_sock_drv.h | 14 ++++++++++++++
 net/xdp/xsk.c              |  5 ++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/net/xdp_sock_drv.h b/include/net/xdp_sock_drv.h
index 4aa031849668..4277b0dcee05 100644
--- a/include/net/xdp_sock_drv.h
+++ b/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)
 {
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 19ac872a6624..86a97da7e50b 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -637,8 +637,11 @@ 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;
-- 
2.27.0