[PATCH liburing] man/io_uring_enter.2: document IORING_OP_SEND_ZC

[Pavel Begunkov <asml.silence@gmail.com>]

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---

Doc writing is not my strongest side, comments are welcome.

 man/io_uring_enter.2 | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/man/io_uring_enter.2 b/man/io_uring_enter.2
index 1a9311e..7fd275c 100644
--- a/man/io_uring_enter.2
+++ b/man/io_uring_enter.2
@@ -1059,6 +1059,50 @@ value being passed in. This request type can be used to either just wake or
 interrupt anyone waiting for completions on the target ring, or it can be used
 to pass messages via the two fields. Available since 5.18.
 
+.TP
+.B IORING_OP_SEND_ZC
+Issue the zerocopy equivalent of a
+.BR send(2)
+system call. It's similar to IORING_OP_SEND, but when the
+.I flags
+field of the
+.I "struct io_uring_cqe"
+contains IORING_CQE_F_MORE, the userspace should expect a second cqe, a.k.a.
+notification, and until then it should not modify data in the buffer. The
+notification will have the same
+.I user_data
+as the first one and its
+.I flags
+field will contain the
+.I IORING_CQE_F_NOTIF
+flag. It's guaranteed that IORING_CQE_F_MORE is set IFF the result is
+non-negative.
+.I fd
+must be set to the socket file descriptor,
+.I addr
+must contain a pointer to the buffer,
+.I len
+denotes the length of the buffer to send, and
+.I msg_flags
+holds the flags associated with the system call. When
+.I addr2
+is non-zero it points to the address of the target with
+.I addr_len
+specifying its size, turning the request into a
+.BR sendto(2)
+system call equivalent.
+
+.B IORING_OP_SEND_ZC
+tries to avoid making intermediate data copies but still may fall back to
+copying. Furthermore, zerocopy is not always faster, especially when the
+per-request payload size is small. The two completion model is needed because
+the kernel might hold on to buffers for a long time, e.g. waiting for a TCP ACK,
+and having a separate cqe for request completions allows the userspace to push
+more data without extra delays. Note, notifications don't guarantee that the
+data has been or will ever be received by the other endpoint.
+
+Available since 5.20.
+
 .PP
 The
 .I flags
-- 
2.37.2