[PATCH v1 1/5] Documentation: Describe dm-user

[Palmer Dabbelt <palmer@dabbelt.com>]

From: Palmer Dabbelt <palmerdabbelt@google.com>

I started by patterning this after the Fuse documentation, which is
located at Documentation/fs/fuse.rst.  There's not a whole lot of that
left, though.

Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

---

This is a work in progress, but nothing in there should be incorrect.
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 Documentation/block/dm-user.rst | 99 +++++++++++++++++++++++++++++++++
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 create mode 100644 Documentation/block/dm-user.rst

diff --git a/Documentation/block/dm-user.rst b/Documentation/block/dm-user.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+=======
+dm-user
+=======
+
+What is dm-user?
+================
+
+dm-user is a device mapper target that allows block accesses to be satisfied by
+an otherwise unprivileged daemon running in userspace.  Conceptually it is
+FUSE, but for block devices as opposed to file systems.
+
+Creating a dm-user Target
+=========================
+
+dm-user is implemented as a Device Mapper target, which allows for various
+device management tasks.  In general dm-user targets function in the same
+fashion as other device-mapper targets, with the exception that dm-user targets
+handle requests via a userspace daemon as opposed to one of various in-kernel
+mechanisms.  As such there is little difference between creating a dm-user
+target and any other device mapper target: the standard device mapper control
+device and ioctl() calls are used to create a table with at least one target of
+the "user" type.  Like all other targets this table entry needs a start/size
+pair.  The additional required argument is the name of the control device that
+will be associated with this target.  Specifically:
+
+````
+user <start sector> <number of sectors> <path to control device>
+````
+
+As a concrete example, the following `dmsetup` invocation will create a new
+device mapper block device available at `/dev/mapper/blk`, consisting entirely
+of a single target which can be controlled via a stream of messages passed over
+`/dev/dm-user/ctl`.
+
+````
+dmsetup create blk <<EOF
+0 1024 user 0 1024 ctl
+EOF
+dmsetup resume blk
+````
+
+Userspace is expected to spin up a daemon to handle those block requests, which
+will block in the meantime.  The various Device Mapper operations should all
+function as they do for other targets.  If the userspace daemon terminates (or
+otherwise closes the control file descriptor) then the kernel will continue to
+queue up BIOs until userspace either starts a new daemon or destroys the
+target.
+
+Userspace may open each control device multiple times, in which case the kernel
+will distribute messages among each file instance.
+
+Writing a dm-user Daemon
+========================
+
+tools/testing/selftests/dm-user contains a handful of test daemons.
+functional/simple-read-all.c is also suitable as an example.
+
+Kernel - userspace interface
+****************************
+
+FIXME: A description of `struct dm_user_message`
+
+Kernel Implementation
+=====================
+
+BIO Lifecycle
+*************
+
+| "dd if=/dev/mapper/user ...             " | dm-user block server
+|                                           |
+|                                           | >sys_read()
+|                                           |  >dev_read()
+|                                           |   [sleep on c->wq]
+|                                           |
+| >sys_read()                               |
+|  [... block and DM layer ... ]            |
+|  >user_map()                              |
+|   [enqueue message]                       |
+|   [wake up c->wq]                         |
+|  <user_map()                              |   [woken up]
+| [sleep on BIO completion]                 |   [copy message to user]
+|                                           |  <dev_read()
+|                                           | <sys_read()
+|                                           |
+|                                           | [obtain request data]
+|                                           |
+|                                           | >sys_write()
+|                                           |  >dev_write()
+|                                           |   [copy message from user]
+|                                           |   [complete BIO]
+|  [woken up on BIO completion]             |  <dev_write()
+| <sys_read()                               | <sys_write()
+|                                           |
+| [write and loop]                          | [loop for more messages]
+
+Locking Scheme
+**************
-- 
2.29.2.454.gaff20da3a2-goog