Re: [PATCH net-next v2 1/2] vsock: report pending receive data to io_uring

AF_VSOCK stream receives never fill msghdr.msg_inq, so io_uring cannot set
IORING_CQE_F_SOCK_NONEMPTY and retries a multishot receive even after the
queue has been drained.

Fill the hint at the common receive exit using the transport callback that
SIOCINQ already uses, and report 1 once the connection is finished so the
caller performs the receive which observes EOF, as TCP does after a FIN.

A vsock loopback ping-pong with io_uring multishot receive drops entries
into __vsock_connectible_recvmsg from 1.97 to 1.00 per delivered message,
and receiver CPU time by about 3% (25 runs of 50000 messages, p=0.006).

Signed-off-by: David Carlier <redacted>
---
 net/vmw_vsock/af_vsock.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index f840498b58af..20d6f9ca6a96 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c

@@ -2543,6 +2543,35 @@ static int __vsock_seqpacket_recvmsg(struct sock *sk, struct msghdr *msg,
 	return err;
 }
 
+/* Bytes a following receive can consume, 1 if it would only see EOF, or -1
+ * if the transport cannot tell.
+ *
+ * Called under the socket lock after a nonnegative stream receive, so a NULL
+ * transport implies SOCK_DONE.
+ */
+static int vsock_stream_inq_hint(struct sock *sk)
+{
+	struct vsock_sock *vsk = vsock_sk(sk);
+	s64 data;
+
+	if ((sk->sk_shutdown & RCV_SHUTDOWN) || !vsk->transport ||
+	    (sock_flag(sk, SOCK_DONE) && sk->sk_state != TCP_ESTABLISHED))
+		return 1;
+
+	data = vsock_stream_has_data(vsk);
+	if (data < 0)
+		return -1;
+	if (data > 0)
+		return min_t(s64, data, INT_MAX);
+
+	/* Empty but finished: keep the caller reading so it sees EOF. */
+	if (sock_flag(sk, SOCK_DONE) ||
+	    (READ_ONCE(vsk->peer_shutdown) & SEND_SHUTDOWN))
+		return 1;
+
+	return 0;
+}
+
 int
 __vsock_connectible_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 			    int flags)

@@ -2606,6 +2635,12 @@ __vsock_connectible_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 		err = __vsock_seqpacket_recvmsg(sk, msg, len, flags);
 
 out:
+	/* Seqpacket has_data counts messages, while io_uring treats msg_inq as