fuse: Add fuse-io-uring design documentation

[Add several documentation updates I had missed after
renaming functions and also fixes 'make htmldocs'.]

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Luis Henriques <luis@igalia.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: bsbernd

Documentation/filesystems/fuse-io-uring.rst

@@ -0,0 +1,99 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=======================================
+FUSE-over-io-uring design documentation
+=======================================
+
+This documentation covers basic details how the fuse
+kernel/userspace communication through io-uring is configured
+and works. For generic details about FUSE see fuse.rst.
+
+This document also covers the current interface, which is
+still in development and might change.
+
+Limitations
+===========
+As of now not all requests types are supported through io-uring, userspace
+is required to also handle requests through /dev/fuse after io-uring setup
+is complete. Specifically notifications (initiated from the daemon side)
+and interrupts.
+
+Fuse io-uring configuration
+===========================
+
+Fuse kernel requests are queued through the classical /dev/fuse
+read/write interface - until io-uring setup is complete.
+
+In order to set up fuse-over-io-uring fuse-server (user-space)
+needs to submit SQEs (opcode = IORING_OP_URING_CMD) to the /dev/fuse
+connection file descriptor. Initial submit is with the sub command
+FUSE_URING_REQ_REGISTER, which will just register entries to be
+available in the kernel.
+
+Once at least one entry per queue is submitted, kernel starts
+to enqueue to ring queues.
+Note, every CPU core has its own fuse-io-uring queue.
+Userspace handles the CQE/fuse-request and submits the result as
+subcommand FUSE_URING_REQ_COMMIT_AND_FETCH - kernel completes
+the requests and also marks the entry available again. If there are
+pending requests waiting the request will be immediately submitted
+to the daemon again.
+
+Initial SQE
+-----------::
+
+ |                                    |  FUSE filesystem daemon
+ |                                    |
+ |                                    |  >io_uring_submit()
+ |                                    |   IORING_OP_URING_CMD /
+ |                                    |   FUSE_URING_CMD_REGISTER
+ |                                    |  [wait cqe]
+ |                                    |   >io_uring_wait_cqe() or
+ |                                    |   >io_uring_submit_and_wait()
+ |                                    |
+ |  >fuse_uring_cmd()                 |
+ |   >fuse_uring_register()           |
+
+
+Sending requests with CQEs
+--------------------------::
+
+ |                                           |  FUSE filesystem daemon
+ |                                           |  [waiting for CQEs]
+ |  "rm /mnt/fuse/file"                      |
+ |                                           |
+ |  >sys_unlink()                            |
+ |    >fuse_unlink()                         |
+ |      [allocate request]                   |
+ |      >fuse_send_one()                     |
+ |        ...                                |
+ |       >fuse_uring_queue_fuse_req          |
+ |        [queue request on fg queue]        |
+ |         >fuse_uring_add_req_to_ring_ent() |
+ |         ...                               |
+ |          >fuse_uring_copy_to_ring()       |
+ |          >io_uring_cmd_done()             |
+ |       >request_wait_answer()              |
+ |         [sleep on req->waitq]             |
+ |                                           |  [receives and handles CQE]
+ |                                           |  [submit result and fetch next]
+ |                                           |  >io_uring_submit()
+ |                                           |   IORING_OP_URING_CMD/
+ |                                           |   FUSE_URING_CMD_COMMIT_AND_FETCH
+ |  >fuse_uring_cmd()                        |
+ |   >fuse_uring_commit_fetch()              |
+ |    >fuse_uring_commit()                   |
+ |     >fuse_uring_copy_from_ring()          |
+ |      [ copy the result to the fuse req]   |
+ |     >fuse_uring_req_end()                 |
+ |      >fuse_request_end()                  |
+ |       [wake up req->waitq]                |
+ |    >fuse_uring_next_fuse_req              |
+ |       [wait or handle next req]           |
+ |                                           |
+ |       [req->waitq woken up]               |
+ |    <fuse_unlink()                         |
+ |  <sys_unlink()                            |
+
+
+

Documentation/filesystems/index.rst

@@ -98,6 +98,7 @@ Documentation for filesystem implementations.
    hpfs
    fuse
    fuse-io
+   fuse-io-uring
    inotify
    isofs
    nilfs2