On Sun, Aug 28, 2022 at 12:50:03PM +0800, Ming Lei wrote: > ublk document is missed when merging ublk driver, so add it now. > > Cc: Jonathan Corbet <corbet@xxxxxxx> > Cc: Richard W.M. Jones <rjones@xxxxxxxxxx> > Cc: ZiyangZhang <ZiyangZhang@xxxxxxxxxxxxxxxxx> > Cc: Stefan Hajnoczi <stefanha@xxxxxxxxxx> > Cc: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> > --- > Documentation/block/ublk.rst | 203 +++++++++++++++++++++++++++++++++++ > 1 file changed, 203 insertions(+) > create mode 100644 Documentation/block/ublk.rst Thanks for preparing this. As you know I had a lot of trouble writing the NBD ublk daemon and this would have helped. TBH I would suggest anyone trying to write a ublk daemon just looks at this source: https://gitlab.com/rwmjones/libnbd/-/tree/nbdublk/ublk > diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst > new file mode 100644 > index 000000000000..9e8f7ba518a3 > --- /dev/null > +++ b/Documentation/block/ublk.rst > @@ -0,0 +1,203 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +========================================== > +Userspace block device driver(ublk driver) > +========================================== > + > +Overview > +======== > + > +ublk is one generic framework for implementing block device logic from "one generic framework" - probably better to say "a generic framework ..." > +userspace. It is very helpful to move virtual block drivers into userspace, > +such as loop, nbd and similar block drivers. It can help to implement new > +virtual block device, such as ublk-qcow2, and there was several attempts > +of implementing qcow2 driver in kernel. On the general topic of this, the qemu developers would greatly prefer that there are not multiple qcow2 implementations. I believe the plan is to modify qemu-storage-daemon (a daemon containing the qemu block layer) to implement ublk. I don't think you really need to mention qcow2 though since it'll be implemented. > +ublk block device(``/dev/ublkb*``) is added by ublk driver. Any IO request > +submitted to ublk device will be forwarded to ublk's userspace part( Add a space between "part" and "("? > +ublksrv [1]), and after the IO is handled by ublksrv, the result is > +committed back to ublk driver, then ublk IO request can be completed. With > +this way, any specific IO handling logic is totally done inside ublksrv, > +and ublk driver doe _not_ handle any device specific IO logic, such as does > +loop's IO handling, NBD's IO communication, or qcow2's IO mapping, ... > > +/dev/ublkbN is driven by blk-mq request based driver, each request is > +assigned by one queue wide unique tag. ublksrv 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 io_uring passthrough command can get > +better IOPS than block IO. So ublk is one high performance implementation > +of userspace block device. Not only IO request communication is done by > +io_uring, but also the preferred IO handling in ublksrv is io_uring based > +approach too. > + > +ublk provides control interface to set/get ublk block device parameters, and > +the interface is extendable and kabi compatible, so basically any ublk request > +queue's parameter or ublk generic feature parameters can be set/get via this > +extendable interface. So ublk is generic userspace block device framework, such > +as, it is easy to setup one ublk device with specified block parameters from > +userspace. > + > +How to use ublk > +=============== > + > +After building ublksrv[1], ublk block device(``/dev/ublkb*``) can be added > +and deleted by the utility, then existed block IO applications can talk with existing > +it. > + > +See usage details in README[2] of ublksrv, for example of ublk-loop: > + > +- add ublk device: > + ublk add -t loop -f ublk-loop.img > + > +- use it: > + mkfs.xfs /dev/ublkb0 > + mount /dev/ublkb0 /mnt > + .... # all IOs are handled by io_uring!!! > + umount /mnt > + > +- get ublk dev info: > + ublk list > + > +- delete ublk device > + ublk del -a > + ublk del -n $ublk_dev_id > + > +Design > +====== > + > +Control plane > +------------- > + > +ublk driver provides global misc device node(``/dev/ublk-control``) for Space between "node" and "(". There are a few more of these below. > +managing and controlling ublk devices with help of several control commands: > + > +- UBLK_CMD_ADD_DEV > + Add one ublk char device(``/dev/ublkc*``) which is talked with ublksrv wrt. > + IO command communication. Basic device info is sent together with this > + command, see UAPI structure of ublksrv_ctrl_dev_info, such as nr_hw_queues, > + queue_depth, and max IO request buffer size, which info is negotiated with > + ublk driver and sent back to ublksrv. After this command is completed, the > + basic device info can't be changed any more. > + > +- UBLK_CMD_SET_PARAMS / UBLK_CMD_GET_PARAMS > + Set or get ublk device's parameters, which can be generic feature related, > + or request queue limit related, but can't be IO logic specific, cause ublk > + driver does not handle any IO logic. This command has to be sent before > + sending UBLK_CMD_START_DEV. > + > +- UBLK_CMD_START_DEV > + After ublksrv prepares userspace resource such as, creating per-queue > + pthread & io_ruing for handling ublk IO, this command is set for ublk set -> sent > + driver to allocate & expose /dev/ublkb*. Parameters set via > + UBLK_CMD_SET_PARAMS are applied for creating /dev/ublkb*. Is this command synchronous? ie. When it completes, is /dev/ublkb* definitely present in the /dev filesystem? (I'm going to guess this depends on something complicated about udevd). > +- UBLK_CMD_STOP_DEV > + Quiesce IO on /dev/ublkb* and delete the disk. After this command returns, > + ublksrv can release resource, such as destroy per-queue pthread & io_uring > + for handling io command. > + > +- UBLK_CMD_DEL_DEV > + Delete /dev/ublkc*. After this command returns, the allocated ublk device > + number can be reused. > + > +- UBLK_CMD_GET_QUEUE_AFFINITY > + After /dev/ublkc is added, ublk driver creates block layer tagset, so each > + queue's affinity info is available, ublksrv sends UBLK_CMD_GET_QUEUE_AFFINITY > + to retrieve queue affinity info, so ublksrv can setup 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 retrieve device info of ublksrv_ctrl_dev_info. And it is ublksrv's > + responsibility to save IO target specific info in userspace. > + > +Data plane > +---------- > + > +ublksrv needs to create per-queue IO pthread & io_uring for handling IO > +command (io_uring passthrough command), and the per-queue IO pthread > +focuses on IO handling and shouldn't handle any control & management > +task. > + > +ublksrv's IO is assigned by one 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. One fixed mmaped area(array) on /dev/ublkc* is provided for > +exporting IO info to ublksrv, 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. > + > +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 ublksrv IO pthread for fetching future coming IO request > + issued to /dev/ublkb*. This command is just sent once from ublksrv IO > + pthread for ublk driver to setup IO forward environment. > + > +- UBLK_IO_COMMIT_AND_FETCH_REQ > + After one IO request is issued to /dev/ublkb*, ublk driver stores this > + 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 ulksrv gets the IO > + notification via io_uring. > + > + After ublksrv handles this IO, this IO's result is committed back to ublk > + driver by sending UBLK_IO_COMMIT_AND_FETCH_REQ back. Once ublkdrv received > + this command, it parses the IO result and complete the IO request to > + /dev/ublkb*. Meantime setup environment for fetching future IO request > + with this IO tag. So UBLK_IO_COMMIT_AND_FETCH_REQ is reused for both > + fetching request and committing back IO result. > + > +- UBLK_IO_NEED_GET_DATA > + ublksrv pre-allocates IO buffer for each IO at default, any new project at default -> by default > + should use this IO buffer to communicate with ublk driver. But existed existed -> existing > + project may not work or be changed to in this way, so add this command > + to provide chance for userspace to use its existed buffer for handling existed -> existing > + IO. > + > +- data copy between ublkserv IO buffer and ublk block IO request > + ublk driver needs to copy ublk block IO request pages into ublksrv buffer > + (pages) first for WRITE before notifying ublksrv of the coming IO, so > + ublksrv can hanldle WRITE request. > + > + After ublksrv handles READ request and sends UBLK_IO_COMMIT_AND_FETCH_REQ > + to ublksrv, ublkdrv needs to copy read ublksrv buffer(pages) to the ublk > + IO request pages. > + > +Future development > +================== > + > +Container-ware ublk deivice Container-aware ublk device > +--------------------------- > + > +ublk driver doesn't handle any IO logic, and its function is well defined > +so far, and very limited userspace interfaces are needed, and each one is > +well defined too, then it is very likely to make ublk device one > +container-ware block device in future, as Stefan Hajnoczi suggested[3], by > +removing ADMIN privilege. Is it advisable for non-root to be able create arbitrary /dev devices? It sounds like a security nightmare because you're exposing potentially any arbitrary, malicious filesystem to the kernel to parse. > +Zero copy > +--------- > + > +Wrt. zero copy support, it is one generic requirement for nbd, fuse or > +similar drivers, one problem Xiaoguang mentioned is that pages mapped to > +userspace can't be remapped any more in kernel with existed mm interfaces, > +and it can be involved when submitting direct IO to /dev/ublkb*. Also > +Xiaoguang reported that big request may benefit from zero copy a lot, > +such as >= 256KB IO. > + > + > +References > +========== > + > +[1] https://github.com/ming1/ubdsrv > + > +[2] https://github.com/ming1/ubdsrv/blob/master/README > + > +[3] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/ > -- > 2.31.1 Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com nbdkit - Flexible, fast NBD server with plugins https://gitlab.com/nbdkit/nbdkit
