--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+
+===========================================
+Userspace block device driver (ublk driver)
+===========================================
+
+Overview
+========
+
+ublk is a generic framework for implementing block device logic from userspace.
+The motivation behind it is that moving virtual block drivers into userspace,
+such as loop, nbd and similar can be very helpful. It can help to implement
+new virtual block device such as ublk-qcow2 (there are several attempts of
+implementing qcow2 driver in kernel).
+
+Userspace block devices are attractive because:
+
+- They can be written many programming languages.
+- They can use libraries that are not available in the kernel.
+- They can be debugged with tools familiar to application developers.
+- Crashes do not kernel panic the machine.
+- Bugs are likely to have a lower security impact than bugs in kernel
+ code.
+- They can be installed and updated independently of the kernel.
+- They can be used to simulate block device easily with user specified
+ parameters/setting for test/debug purpose
+
+ublk block device (``/dev/ublkb*``) is added by ublk driver. Any IO request
+on the device will be forwarded to ublk userspace program. For convenience,
+in this document, ``ublk server`` refers to generic ublk userspace
+program. ``ublksrv`` [#userspace]_ is one of such implementation. It
+provides ``libublksrv`` [#userspace_lib]_ library for developing specific
+user block device conveniently, while also generic type block device is
+included, such as loop and null. Richard W.M. Jones wrote userspace nbd device
+``nbdublk`` [#userspace_nbdublk]_ based on ``libublksrv`` [#userspace_lib]_.
+
+After the IO is handled by userspace, the result is committed back to the
+driver, thus completing the request cycle. This way, any specific IO handling
+logic is totally done by userspace, such as loop's IO handling, NBD's IO
+communication, or qcow2's IO mapping.
+
+``/dev/ublkb*`` is driven by blk-mq request-based driver. Each request is
+assigned by one queue wide unique tag. ublk server assigns unique tag to each
+IO too, which is 1:1 mapped with IO of ``/dev/ublkb*``.
+
+Both the IO request forward and IO handling result committing are done via
+``io_uring`` passthrough command; that is why ublk is also one io_uring based
+block driver. It has been observed that using io_uring passthrough command can
+give better IOPS than block IO; which is why ublk is one of high performance
+implementation of userspace block device: not only IO request communication is
+done by io_uring, but also the preferred IO handling in ublk server is io_uring
+based approach too.
+
+ublk provides control interface to set/get ublk block device parameters.
+The interface is extendable and kabi compatible: basically any ublk request
+queue's parameter or ublk generic feature parameters can be set/get via the
+interface. Thus, ublk is generic userspace block device framework.
+For example, it is easy to setup a ublk device with specified block
+parameters from userspace.
+
+Using ublk
+==========
+
+ublk requires userspace ublk server to handle real block device logic.
+
+Below is example of using ``ublksrv`` to provide ublk-based loop device.
+
+- add a device::
+
+ ublk add -t loop -f ublk-loop.img
+
+- format with xfs, then use it::
+
+ mkfs.xfs /dev/ublkb0
+ mount /dev/ublkb0 /mnt
+ # do anything. all IOs are handled by io_uring
+ ...
+ umount /mnt
+
+- list the devices with their info::
+
+ ublk list
+
+- delete the device::
+
+ ublk del -a
+ ublk del -n $ublk_dev_id
+
+See usage details in README of ``ublksrv`` [#userspace_readme]_.
+
+Design
+======
+
+Control plane
+-------------
+
+ublk driver provides global misc device node (``/dev/ublk-control``) for
+managing and controlling ublk devices with help of several control commands:
+
+- ``UBLK_CMD_ADD_DEV``
+
+ Add a ublk char device (``/dev/ublkc*``) which is talked with ublk server
+ WRT IO command communication. Basic device info is sent together with this
+ command. It sets UAPI structure of ``ublksrv_ctrl_dev_info``,
+ such as ``nr_hw_queues``, ``queue_depth``, and max IO request buffer size,
+ for which the info is negotiated with the driver and sent back to the server.
+ When this command is completed, the basic device info is immutable.
+
+- ``UBLK_CMD_SET_PARAMS`` / ``UBLK_CMD_GET_PARAMS``
+
+ Set or get parameters of the device, which can be either generic feature
+ related, or request queue limit related, but can't be IO logic specific,
+ because the driver does not handle any IO logic. This command has to be
+ sent before sending ``UBLK_CMD_START_DEV``.
+
+- ``UBLK_CMD_START_DEV``
+
+ After the server prepares userspace resources (such as creating per-queue
+ pthread & io_uring for handling ublk IO), this command is sent to the
+ driver for allocating & exposing ``/dev/ublkb*``. Parameters set via
+ ``UBLK_CMD_SET_PARAMS`` are applied for creating the device.
+
+- ``UBLK_CMD_STOP_DEV``
+
+ Halt IO on ``/dev/ublkb*`` and remove the device. When this command returns,
+ ublk server will release resources (such as destroying per-queue pthread &
+ io_uring).
+
+- ``UBLK_CMD_DEL_DEV``
+
+ Remove ``/dev/ublkc*``. When this command returns, the allocated ublk device
+ number can be reused.
+
+- ``UBLK_CMD_GET_QUEUE_AFFINITY``
+
+ When ``/dev/ublkc`` is added, the driver creates block layer tagset, so
+ that each queue's affinity info is available. The server sends
+ ``UBLK_CMD_GET_QUEUE_AFFINITY`` to retrieve queue affinity info. It can
+ set up the per-queue context efficiently, such as bind affine CPUs with IO
+ pthread and try to allocate buffers in IO thread context.
+
+- ``UBLK_CMD_GET_DEV_INFO``
+
+ For retrieving device info via ``ublksrv_ctrl_dev_info``. It is the server's
+ responsibility to save IO target specific info in userspace.
+
+Data plane
+----------
+
+ublk server needs to create per-queue IO pthread & io_uring for handling IO
+commands via io_uring passthrough. The per-queue IO pthread
+focuses on IO handling and shouldn't handle any control & management
+tasks.
+
+The's IO is assigned by a unique tag, which is 1:1 mapping with IO
+request of ``/dev/ublkb*``.
+
+UAPI structure of ``ublksrv_io_desc`` is defined for describing each IO from
+the driver. A fixed mmaped area (array) on ``/dev/ublkc*`` is provided for
+exporting IO info to the server; such as IO offset, length, OP/flags and
+buffer address. Each ``ublksrv_io_desc`` instance can be indexed via queue id
+and IO tag directly.
+
+The following IO commands are communicated via io_uring passthrough command,
+and each command is only for forwarding the IO and committing the result
+with specified IO tag in the command data:
+
+- ``UBLK_IO_FETCH_REQ``
+
+ Sent from the server IO pthread for fetching future incoming IO requests
+ destined to ``/dev/ublkb*``. This command is sent only once from the server
+ IO pthread for ublk driver to setup IO forward environment.
+
+- ``UBLK_IO_COMMIT_AND_FETCH_REQ``
+
+ When an IO request is destined to ``/dev/ublkb*``, the driver stores
+ the IO's ``ublksrv_io_desc`` to the specified mapped area; then the
+ previous received IO command of this IO tag (either ``UBLK_IO_FETCH_REQ``
+ or ``UBLK_IO_COMMIT_AND_FETCH_REQ)`` is completed, so the server gets
+ the IO notification via io_uring.
+
+ After the server handles the IO, its result is committed back to the
+ driver by sending ``UBLK_IO_COMMIT_AND_FETCH_REQ`` back. Once ublkdrv
+ received this command, it parses the result and complete the request to
+ ``/dev/ublkb*``. In the meantime setup environment for fetching future
+ requests with the same IO tag. That is, ``UBLK_IO_COMMIT_AND_FETCH_REQ``
+ is reused for both fetching request and committing back IO result.
+
+- ``UBLK_IO_NEED_GET_DATA``
+
+ With ``UBLK_F_NEED_GET_DATA`` enabled, the WRITE request will be firstly
+ issued to ublk server without data copy. Then, IO backend of ublk server
+ receives the request and it can allocate data buffer and embed its addr
+ inside this new io command. After the kernel driver gets the command,
+ data copy is done from request pages to this backend's buffer. Finally,
+ backend receives the request again with data to be written and it can
+ truly handle the request.
+
+ ``UBLK_IO_NEED_GET_DATA`` adds one additional round-trip and one
+ io_uring_enter() syscall. Any user thinks that it may lower performance
+ should not enable UBLK_F_NEED_GET_DATA. ublk server pre-allocates IO
+ buffer for each IO by default. Any new project should try to use this
+ buffer to communicate with ublk driver. However, existing project may
+ break or not able to consume the new buffer interface; that's why this
+ command is added for backwards compatibility so that existing projects
+ can still consume existing buffers.
+
+- data copy between ublk server IO buffer and ublk block IO request
+
+ The driver needs to copy the block IO request pages into the server buffer
+ (pages) first for WRITE before notifying the server of the coming IO, so
+ that the server can handle WRITE request.
+
+ When the server handles READ request and sends
+ ``UBLK_IO_COMMIT_AND_FETCH_REQ`` to the server, ublkdrv needs to copy
+ the server buffer (pages) read to the IO request pages.
+
+Future development
+==================
+
+Container-aware ublk deivice
+----------------------------
+
+ublk driver doesn't handle any IO logic. Its function is well defined
+for now and very limited userspace interfaces are needed, which is also
+well defined too. It is possible to make ublk devices container-aware block
+devices in future as Stefan Hajnoczi suggested [#stefan]_, by removing
+ADMIN privilege.
+
+Zero copy
+---------
+
+Zero copy is a generic requirement for nbd, fuse or similar drivers. A
+problem [#xiaoguang]_ Xiaoguang mentioned is that pages mapped to userspace
+can't be remapped any more in kernel with existing mm interfaces. This can
+occurs when destining direct IO to ``/dev/ublkb*``. Also, he reported that
+big requests (IO size >= 256 KB) may benefit a lot from zero copy.
+
+
+References
+==========
+
+.. [#userspace] https://github.com/ming1/ubdsrv
+
+.. [#userspace_lib] https://github.com/ming1/ubdsrv/tree/master/lib
+
+.. [#userspace_nbdublk] https://gitlab.com/rwmjones/libnbd/-/tree/nbdublk
+
+.. [#userspace_readme] https://github.com/ming1/ubdsrv/blob/master/README
+
+.. [#stefan] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/
+
+.. [#xiaoguang] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/
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