fuse: Add fuse-io-uring design documentation

.. 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()                            |

   hpfs

   fuse

   fuse-io

   fuse-io-uring

   inotify

   isofs

   nilfs2