1.. SPDX-License-Identifier: GPL-2.0 2 3=========================================== 4Userspace block device driver (ublk driver) 5=========================================== 6 7Overview 8======== 9 10ublk is a generic framework for implementing block device logic from userspace. 11The motivation behind it is that moving virtual block drivers into userspace, 12such as loop, nbd and similar can be very helpful. It can help to implement 13new virtual block device such as ublk-qcow2 (there are several attempts of 14implementing qcow2 driver in kernel). 15 16Userspace block devices are attractive because: 17 18- They can be written many programming languages. 19- They can use libraries that are not available in the kernel. 20- They can be debugged with tools familiar to application developers. 21- Crashes do not kernel panic the machine. 22- Bugs are likely to have a lower security impact than bugs in kernel 23 code. 24- They can be installed and updated independently of the kernel. 25- They can be used to simulate block device easily with user specified 26 parameters/setting for test/debug purpose 27 28ublk block device (``/dev/ublkb*``) is added by ublk driver. Any IO request 29on the device will be forwarded to ublk userspace program. For convenience, 30in this document, ``ublk server`` refers to generic ublk userspace 31program. ``ublksrv`` [#userspace]_ is one of such implementation. It 32provides ``libublksrv`` [#userspace_lib]_ library for developing specific 33user block device conveniently, while also generic type block device is 34included, such as loop and null. Richard W.M. Jones wrote userspace nbd device 35``nbdublk`` [#userspace_nbdublk]_ based on ``libublksrv`` [#userspace_lib]_. 36 37After the IO is handled by userspace, the result is committed back to the 38driver, thus completing the request cycle. This way, any specific IO handling 39logic is totally done by userspace, such as loop's IO handling, NBD's IO 40communication, or qcow2's IO mapping. 41 42``/dev/ublkb*`` is driven by blk-mq request-based driver. Each request is 43assigned by one queue wide unique tag. ublk server assigns unique tag to each 44IO too, which is 1:1 mapped with IO of ``/dev/ublkb*``. 45 46Both the IO request forward and IO handling result committing are done via 47``io_uring`` passthrough command; that is why ublk is also one io_uring based 48block driver. It has been observed that using io_uring passthrough command can 49give better IOPS than block IO; which is why ublk is one of high performance 50implementation of userspace block device: not only IO request communication is 51done by io_uring, but also the preferred IO handling in ublk server is io_uring 52based approach too. 53 54ublk provides control interface to set/get ublk block device parameters. 55The interface is extendable and kabi compatible: basically any ublk request 56queue's parameter or ublk generic feature parameters can be set/get via the 57interface. Thus, ublk is generic userspace block device framework. 58For example, it is easy to setup a ublk device with specified block 59parameters from userspace. 60 61Using ublk 62========== 63 64ublk requires userspace ublk server to handle real block device logic. 65 66Below is example of using ``ublksrv`` to provide ublk-based loop device. 67 68- add a device:: 69 70 ublk add -t loop -f ublk-loop.img 71 72- format with xfs, then use it:: 73 74 mkfs.xfs /dev/ublkb0 75 mount /dev/ublkb0 /mnt 76 # do anything. all IOs are handled by io_uring 77 ... 78 umount /mnt 79 80- list the devices with their info:: 81 82 ublk list 83 84- delete the device:: 85 86 ublk del -a 87 ublk del -n $ublk_dev_id 88 89See usage details in README of ``ublksrv`` [#userspace_readme]_. 90 91Design 92====== 93 94Control plane 95------------- 96 97ublk driver provides global misc device node (``/dev/ublk-control``) for 98managing and controlling ublk devices with help of several control commands: 99 100- ``UBLK_CMD_ADD_DEV`` 101 102 Add a ublk char device (``/dev/ublkc*``) which is talked with ublk server 103 WRT IO command communication. Basic device info is sent together with this 104 command. It sets UAPI structure of ``ublksrv_ctrl_dev_info``, 105 such as ``nr_hw_queues``, ``queue_depth``, and max IO request buffer size, 106 for which the info is negotiated with the driver and sent back to the server. 107 When this command is completed, the basic device info is immutable. 108 109- ``UBLK_CMD_SET_PARAMS`` / ``UBLK_CMD_GET_PARAMS`` 110 111 Set or get parameters of the device, which can be either generic feature 112 related, or request queue limit related, but can't be IO logic specific, 113 because the driver does not handle any IO logic. This command has to be 114 sent before sending ``UBLK_CMD_START_DEV``. 115 116- ``UBLK_CMD_START_DEV`` 117 118 After the server prepares userspace resources (such as creating per-queue 119 pthread & io_uring for handling ublk IO), this command is sent to the 120 driver for allocating & exposing ``/dev/ublkb*``. Parameters set via 121 ``UBLK_CMD_SET_PARAMS`` are applied for creating the device. 122 123- ``UBLK_CMD_STOP_DEV`` 124 125 Halt IO on ``/dev/ublkb*`` and remove the device. When this command returns, 126 ublk server will release resources (such as destroying per-queue pthread & 127 io_uring). 128 129- ``UBLK_CMD_DEL_DEV`` 130 131 Remove ``/dev/ublkc*``. When this command returns, the allocated ublk device 132 number can be reused. 133 134- ``UBLK_CMD_GET_QUEUE_AFFINITY`` 135 136 When ``/dev/ublkc`` is added, the driver creates block layer tagset, so 137 that each queue's affinity info is available. The server sends 138 ``UBLK_CMD_GET_QUEUE_AFFINITY`` to retrieve queue affinity info. It can 139 set up the per-queue context efficiently, such as bind affine CPUs with IO 140 pthread and try to allocate buffers in IO thread context. 141 142- ``UBLK_CMD_GET_DEV_INFO`` 143 144 For retrieving device info via ``ublksrv_ctrl_dev_info``. It is the server's 145 responsibility to save IO target specific info in userspace. 146 147- ``UBLK_CMD_GET_DEV_INFO2`` 148 Same purpose with ``UBLK_CMD_GET_DEV_INFO``, but ublk server has to 149 provide path of the char device of ``/dev/ublkc*`` for kernel to run 150 permission check, and this command is added for supporting unprivileged 151 ublk device, and introduced with ``UBLK_F_UNPRIVILEGED_DEV`` together. 152 Only the user owning the requested device can retrieve the device info. 153 154 How to deal with userspace/kernel compatibility: 155 156 1) if kernel is capable of handling ``UBLK_F_UNPRIVILEGED_DEV`` 157 158 If ublk server supports ``UBLK_F_UNPRIVILEGED_DEV``: 159 160 ublk server should send ``UBLK_CMD_GET_DEV_INFO2``, given anytime 161 unprivileged application needs to query devices the current user owns, 162 when the application has no idea if ``UBLK_F_UNPRIVILEGED_DEV`` is set 163 given the capability info is stateless, and application should always 164 retrieve it via ``UBLK_CMD_GET_DEV_INFO2`` 165 166 If ublk server doesn't support ``UBLK_F_UNPRIVILEGED_DEV``: 167 168 ``UBLK_CMD_GET_DEV_INFO`` is always sent to kernel, and the feature of 169 UBLK_F_UNPRIVILEGED_DEV isn't available for user 170 171 2) if kernel isn't capable of handling ``UBLK_F_UNPRIVILEGED_DEV`` 172 173 If ublk server supports ``UBLK_F_UNPRIVILEGED_DEV``: 174 175 ``UBLK_CMD_GET_DEV_INFO2`` is tried first, and will be failed, then 176 ``UBLK_CMD_GET_DEV_INFO`` needs to be retried given 177 ``UBLK_F_UNPRIVILEGED_DEV`` can't be set 178 179 If ublk server doesn't support ``UBLK_F_UNPRIVILEGED_DEV``: 180 181 ``UBLK_CMD_GET_DEV_INFO`` is always sent to kernel, and the feature of 182 ``UBLK_F_UNPRIVILEGED_DEV`` isn't available for user 183 184- ``UBLK_CMD_START_USER_RECOVERY`` 185 186 This command is valid if ``UBLK_F_USER_RECOVERY`` feature is enabled. This 187 command is accepted after the old process has exited, ublk device is quiesced 188 and ``/dev/ublkc*`` is released. User should send this command before he starts 189 a new process which re-opens ``/dev/ublkc*``. When this command returns, the 190 ublk device is ready for the new process. 191 192- ``UBLK_CMD_END_USER_RECOVERY`` 193 194 This command is valid if ``UBLK_F_USER_RECOVERY`` feature is enabled. This 195 command is accepted after ublk device is quiesced and a new process has 196 opened ``/dev/ublkc*`` and get all ublk queues be ready. When this command 197 returns, ublk device is unquiesced and new I/O requests are passed to the 198 new process. 199 200- user recovery feature description 201 202 Three new features are added for user recovery: ``UBLK_F_USER_RECOVERY``, 203 ``UBLK_F_USER_RECOVERY_REISSUE``, and ``UBLK_F_USER_RECOVERY_FAIL_IO``. To 204 enable recovery of ublk devices after the ublk server exits, the ublk server 205 should specify the ``UBLK_F_USER_RECOVERY`` flag when creating the device. The 206 ublk server may additionally specify at most one of 207 ``UBLK_F_USER_RECOVERY_REISSUE`` and ``UBLK_F_USER_RECOVERY_FAIL_IO`` to 208 modify how I/O is handled while the ublk server is dying/dead (this is called 209 the ``nosrv`` case in the driver code). 210 211 With just ``UBLK_F_USER_RECOVERY`` set, after one ubq_daemon(ublk server's io 212 handler) is dying, ublk does not delete ``/dev/ublkb*`` during the whole 213 recovery stage and ublk device ID is kept. It is ublk server's 214 responsibility to recover the device context by its own knowledge. 215 Requests which have not been issued to userspace are requeued. Requests 216 which have been issued to userspace are aborted. 217 218 With ``UBLK_F_USER_RECOVERY_REISSUE`` additionally set, after one ubq_daemon 219 (ublk server's io handler) is dying, contrary to ``UBLK_F_USER_RECOVERY``, 220 requests which have been issued to userspace are requeued and will be 221 re-issued to the new process after handling ``UBLK_CMD_END_USER_RECOVERY``. 222 ``UBLK_F_USER_RECOVERY_REISSUE`` is designed for backends who tolerate 223 double-write since the driver may issue the same I/O request twice. It 224 might be useful to a read-only FS or a VM backend. 225 226 With ``UBLK_F_USER_RECOVERY_FAIL_IO`` additionally set, after the ublk server 227 exits, requests which have issued to userspace are failed, as are any 228 subsequently issued requests. Applications continuously issuing I/O against 229 devices with this flag set will see a stream of I/O errors until a new ublk 230 server recovers the device. 231 232Unprivileged ublk device is supported by passing ``UBLK_F_UNPRIVILEGED_DEV``. 233Once the flag is set, all control commands can be sent by unprivileged 234user. Except for command of ``UBLK_CMD_ADD_DEV``, permission check on 235the specified char device(``/dev/ublkc*``) is done for all other control 236commands by ublk driver, for doing that, path of the char device has to 237be provided in these commands' payload from ublk server. With this way, 238ublk device becomes container-ware, and device created in one container 239can be controlled/accessed just inside this container. 240 241Data plane 242---------- 243 244ublk server needs to create per-queue IO pthread & io_uring for handling IO 245commands via io_uring passthrough. The per-queue IO pthread 246focuses on IO handling and shouldn't handle any control & management 247tasks. 248 249The's IO is assigned by a unique tag, which is 1:1 mapping with IO 250request of ``/dev/ublkb*``. 251 252UAPI structure of ``ublksrv_io_desc`` is defined for describing each IO from 253the driver. A fixed mmapped area (array) on ``/dev/ublkc*`` is provided for 254exporting IO info to the server; such as IO offset, length, OP/flags and 255buffer address. Each ``ublksrv_io_desc`` instance can be indexed via queue id 256and IO tag directly. 257 258The following IO commands are communicated via io_uring passthrough command, 259and each command is only for forwarding the IO and committing the result 260with specified IO tag in the command data: 261 262- ``UBLK_IO_FETCH_REQ`` 263 264 Sent from the server IO pthread for fetching future incoming IO requests 265 destined to ``/dev/ublkb*``. This command is sent only once from the server 266 IO pthread for ublk driver to setup IO forward environment. 267 268- ``UBLK_IO_COMMIT_AND_FETCH_REQ`` 269 270 When an IO request is destined to ``/dev/ublkb*``, the driver stores 271 the IO's ``ublksrv_io_desc`` to the specified mapped area; then the 272 previous received IO command of this IO tag (either ``UBLK_IO_FETCH_REQ`` 273 or ``UBLK_IO_COMMIT_AND_FETCH_REQ)`` is completed, so the server gets 274 the IO notification via io_uring. 275 276 After the server handles the IO, its result is committed back to the 277 driver by sending ``UBLK_IO_COMMIT_AND_FETCH_REQ`` back. Once ublkdrv 278 received this command, it parses the result and complete the request to 279 ``/dev/ublkb*``. In the meantime setup environment for fetching future 280 requests with the same IO tag. That is, ``UBLK_IO_COMMIT_AND_FETCH_REQ`` 281 is reused for both fetching request and committing back IO result. 282 283- ``UBLK_IO_NEED_GET_DATA`` 284 285 With ``UBLK_F_NEED_GET_DATA`` enabled, the WRITE request will be firstly 286 issued to ublk server without data copy. Then, IO backend of ublk server 287 receives the request and it can allocate data buffer and embed its addr 288 inside this new io command. After the kernel driver gets the command, 289 data copy is done from request pages to this backend's buffer. Finally, 290 backend receives the request again with data to be written and it can 291 truly handle the request. 292 293 ``UBLK_IO_NEED_GET_DATA`` adds one additional round-trip and one 294 io_uring_enter() syscall. Any user thinks that it may lower performance 295 should not enable UBLK_F_NEED_GET_DATA. ublk server pre-allocates IO 296 buffer for each IO by default. Any new project should try to use this 297 buffer to communicate with ublk driver. However, existing project may 298 break or not able to consume the new buffer interface; that's why this 299 command is added for backwards compatibility so that existing projects 300 can still consume existing buffers. 301 302- data copy between ublk server IO buffer and ublk block IO request 303 304 The driver needs to copy the block IO request pages into the server buffer 305 (pages) first for WRITE before notifying the server of the coming IO, so 306 that the server can handle WRITE request. 307 308 When the server handles READ request and sends 309 ``UBLK_IO_COMMIT_AND_FETCH_REQ`` to the server, ublkdrv needs to copy 310 the server buffer (pages) read to the IO request pages. 311 312Future development 313================== 314 315Zero copy 316--------- 317 318Zero copy is a generic requirement for nbd, fuse or similar drivers. A 319problem [#xiaoguang]_ Xiaoguang mentioned is that pages mapped to userspace 320can't be remapped any more in kernel with existing mm interfaces. This can 321occurs when destining direct IO to ``/dev/ublkb*``. Also, he reported that 322big requests (IO size >= 256 KB) may benefit a lot from zero copy. 323 324 325References 326========== 327 328.. [#userspace] https://github.com/ming1/ubdsrv 329 330.. [#userspace_lib] https://github.com/ming1/ubdsrv/tree/master/lib 331 332.. [#userspace_nbdublk] https://gitlab.com/rwmjones/libnbd/-/tree/nbdublk 333 334.. [#userspace_readme] https://github.com/ming1/ubdsrv/blob/master/README 335 336.. [#stefan] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/ 337 338.. [#xiaoguang] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/ 339