Userspace block driver(ublk)

Introduction

This is the userspace daemon part(ublksrv) of the ublk framework, the other part is ublk driver [#userspace]_ which supports multiple queue.

The two parts communicate by io_uring's IORING_OP_URING_CMD with one per-queue shared cmd buffer for storing io command, and the buffer is read only for ublksrv, each io command can be indexed by io request tag directly, and the command is written by ublk driver, and read by ublksrv after getting notification from ublk driver.

For example, when one READ io request is submitted to ublk block driver, ublk driver stores the io command into cmd buffer first, then completes one IORING_OP_URING_CMD for notifying ublksrv, and the URING_CMD is issued to ublk driver beforehand by ublksrv for getting notification of any new io request, and each URING_CMD is associated with one io request by tag, so depth for URING_CMD is same with queue depth of ublk block device.

After ublksrv gets the io command, it translates and handles the ublk io request, such as, for the ublk-loop target, ublksrv translates the request into same request on another file or disk, like the kernel loop block driver. In ublksrv's implementation, the io is still handled by io_uring, and share same ring with IORING_OP_URING_CMD command. When the target io request is done, the same IORING_OP_URING_CMD is issued to ublk driver for both committing io request result and getting future notification of new io request.

So far, the ublk driver needs to copy io request pages into userspace buffer (pages) first for write before notifying the request to ublksrv, and copy userspace buffer(pages) to the io request pages after ublksrv handles READ. Also looks linux-mm can't support zero copy for this case yet. [2]

More ublk targets will be added with this framework in future even though only ublk-loop and ublk-null are implemented now.

libublksrv is also generated, and it helps to integrate ublk into existed project. One example of demo_null is provided for how to make a ublk device over libublksrv.

Quick start

how to build ublksrv:

autoreconf -i

./configure

make

note: './configure' requires liburing 2.2 package installed, if liburing 2.2 isn't available in your distribution, please configure via the following command, or refer to build_with_liburing_src [6]

PKG_CONFIG_PATH=${LIBURING_DIR} ./configure

CFLAGS="-I${LIBURING_DIR}/src/include" CXXFLAGS="-I${LIBURING_DIR}/src/include" LDFLAGS="-L${LIBURING_DIR}/src"

and LIBURING_DIR points to directory of liburing source code, and liburing needs to be built before running above commands. Also IORING_SETUP_SQE128 has to be supported in the liburing source.

c++20 is required for building ublk utility, but libublksrv and demo_null.c & demo_event.c can be built independently:

#build libublksrv

make -C lib/

#build demo_null && demo_event

make -C lib/

make demo_null demo_event

help

• ublk help

add one ublk-null disk

• ublk add -t null

add one ublk-loop disk

• ublk add -t loop -f /dev/vdb

or

• ublk add -t loop -f 1.img

add one qcow2 disk

• ublk add -t qcow2 -f test.qcow2

note: qcow2 support is experimental, see details in qcow2 status [4] and readme [5]

remove one ublk disk

• ublk del -n 0 #remove /dev/ublkb0
• ublk del -a #remove all ublk devices

list ublk devices

• ublk list
• ublk list -v #with all device info dumped

test

run all built tests

make test T=all

run test group

make test T=null

make test T=loop

make test T=generic

run single test

make test T=generic/001

make test T=null/001

make test T=loop/001 ...

run specified tests or test groups

make test T=generic:loop/001:null

Debug

ublksrv is running as one daemon process, so most of debug messages won't be shown in terminal. If any issue is observed, please collect log via command of "journalctl | grep ublksrvd"

License

nlohmann(include/nlohmann/json.hpp) is from [3], which is covered by MIT license.

The library functions (all code in lib/ directory and include/ublksrv.h) are covered by dual licensed LGPL and MIT, see COPYING.LGPL and LICENSE.

qcow2 target code is covered by GPL-2.0, see COPYING.

All other source code are covered by dual licensed GPL and MIT, see COPYING and LICENSE.

References

[1]	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/block/ublk_drv.c?h=v6.0

[2]	https://lore.kernel.org/all/20220318095531.15479-1-xiaoguang.wang@linux.alibaba.com/

[3]	https://github.com/nlohmann/json

[4]	https://github.com/ming1/ubdsrv/blob/master/qcow2/STATUS.rst

[5]	https://github.com/ming1/ubdsrv/blob/master/qcow2/README.rst

[6]	https://github.com/ming1/ubdsrv/blob/master/build_with_liburing_src