[RFC PATCH bpf-next 4/9] xsk: check need wakeup flag in sendmsg()

[Björn Töpel]

From: Björn Töpel <bjorn.to...@intel.com>

Add a check for need wake up in sendmsg(), so that if a user calls
sendmsg() when no wakeup is needed, do not trigger a wakeup.

To simplify the need wakeup check in the syscall, unconditionally
enable the need wakeup flag for Tx. This has a side-effect for poll();
If poll() is called for a socket without enabled need wakeup, a Tx
wakeup is unconditionally performed.

The wakeup matrix for AF_XDP now looks like:

need wakeup | poll()                 | sendmsg()           | recvmsg()
------------+------------------------+---------------------+---------------------
disabled    | wake Tx                | wake Tx             | nop
enabled     | check flag; wake Tx/Rx | check flag; wake Tx | check flag; wake Rx

Signed-off-by: Björn Töpel <bjorn.to...@intel.com>
---
 net/xdp/xsk.c           |  6 +++++-
 net/xdp/xsk_buff_pool.c | 13 ++++++-------
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 17d51d1a5752..2e5b9f27c7a3 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -465,13 +465,17 @@ static int xsk_sendmsg(struct socket *sock, struct msghdr 
*m, size_t total_len)
        bool need_wait = !(m->msg_flags & MSG_DONTWAIT);
        struct sock *sk = sock->sk;
        struct xdp_sock *xs = xdp_sk(sk);
+       struct xsk_buff_pool *pool;
 
        if (unlikely(!xsk_is_bound(xs)))
                return -ENXIO;
        if (unlikely(need_wait))
                return -EOPNOTSUPP;
 
-       return __xsk_sendmsg(sk);
+       pool = xs->pool;
+       if (pool->cached_need_wakeup & XDP_WAKEUP_TX)
+               return __xsk_sendmsg(sk);
+       return 0;
 }
 
 static int xsk_recvmsg(struct socket *sock, struct msghdr *m, size_t len, int 
flags)
diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
index 64c9e55d4d4e..a4acb5e9576f 100644
--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -144,14 +144,13 @@ static int __xp_assign_dev(struct xsk_buff_pool *pool,
        if (err)
                return err;
 
-       if (flags & XDP_USE_NEED_WAKEUP) {
+       if (flags & XDP_USE_NEED_WAKEUP)
                pool->uses_need_wakeup = true;
-               /* Tx needs to be explicitly woken up the first time.
-                * Also for supporting drivers that do not implement this
-                * feature. They will always have to call sendto().
-                */
-               pool->cached_need_wakeup = XDP_WAKEUP_TX;
-       }
+       /* Tx needs to be explicitly woken up the first time.  Also
+        * for supporting drivers that do not implement this
+        * feature. They will always have to call sendto() or poll().
+        */
+       pool->cached_need_wakeup = XDP_WAKEUP_TX;
 
        dev_hold(netdev);
 
-- 
2.27.0