io_uring/rw: add support for IORING_OP_READ_MULTISHOT

This behaves like IORING_OP_READ, except:

1) It only supports pollable files (eg pipes, sockets, etc). Note that
   for sockets, you probably want to use recv/recvmsg with multishot
   instead.

2) It supports multishot mode, meaning it will repeatedly trigger a
   read and fill a buffer when data is available. This allows similar
   use to recv/recvmsg but on non-sockets, where a single request will
   repeatedly post a CQE whenever data is read from it.

3) Because of #2, it must be used with provided buffers. This is
   uniformly true across any request type that supports multishot and
   transfers data, with the reason being that it's obviously not
   possible to pass in a single buffer for the data, as multiple reads
   may very well trigger before an application has a chance to process
   previous CQEs and the data passed from them.

Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 2 insertions, 0 deletions

diff --git a/io_uring/rw.h b/io_uring/rw.h
index 4b89f9659366..c5aed03d42a4 100644
--- a/io_uring/rw.h
+++ b/io_uring/rw.h
@@ -23,3 +23,5 @@ int io_writev_prep_async(struct io_kiocb *req);
 void io_readv_writev_cleanup(struct io_kiocb *req);
 void io_rw_fail(struct io_kiocb *req);
 void io_req_rw_complete(struct io_kiocb *req, struct io_tw_state *ts);
+int io_read_mshot_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
+int io_read_mshot(struct io_kiocb *req, unsigned int issue_flags);