On Thu, Sep 01, 2022 at 09:30:22PM +0800, Ming Lei wrote: > Add documentation for ublk subsystem. It was supposed to be documented when > merging the driver, but missing at that time. > The subject line should be "Documentation: document ublk" > +.. 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 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 was 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 > +ublk 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 ublk IO and committing IO 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 (>= 256 KB IO) may benefit a lot from zero copy. > + > + I also have minor grammatical fixes: ---- >8 ---- diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst index a102f4c1ab2eb4..2122d1a4a54196 100644 --- a/Documentation/block/ublk.rst +++ b/Documentation/block/ublk.rst @@ -8,9 +8,9 @@ Overview ======== ublk is a generic framework for implementing block device logic from userspace. -The motivation behind it is moving virtual block drivers into 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 was several attempts of +new virtual block device such as ublk-qcow2 (there are several attempts of implementing qcow2 driver in kernel). Userspace block devices are attractive because: @@ -156,13 +156,13 @@ 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 -ublk driver. A fixed mmaped area (array) on ``/dev/ublkc*`` is provided for +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 ublk IO and committing IO result +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`` @@ -233,8 +233,8 @@ 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 (>= 256 KB IO) may benefit a lot from zero copy. +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 Thanks. -- An old man doll... just what I always wanted! - Clara
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