From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2EC88C0502F for ; Sun, 28 Aug 2022 04:50:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231845AbiH1Eu0 (ORCPT ); Sun, 28 Aug 2022 00:50:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38942 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231223AbiH1EuZ (ORCPT ); Sun, 28 Aug 2022 00:50:25 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 971A6BF7B for ; Sat, 27 Aug 2022 21:50:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1661662221; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=Z4N2mShU2F+NTcI5qBpG9QMqojL5Q+6p5Bhy7zouWtE=; b=CwSAUUmrYmrWD6eO17i293Oq2+cvhFjDezpe0sD2N1YAyoCu4l5k5Sy/EsnKMzdW9Nj1bA 9WPYqztpgJrKM38znj9W94utnvCftd6w65yfE70UwGAcIzgX6I7XdKPP9uPPg9b6N08jjo 64D5YQuMND9pOE+Xye2qVAqyg2OpQT0= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-135-NuFv7WgBMoatckQ8KlYUqw-1; Sun, 28 Aug 2022 00:50:17 -0400 X-MC-Unique: NuFv7WgBMoatckQ8KlYUqw-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 650581824634; Sun, 28 Aug 2022 04:50:17 +0000 (UTC) Received: from localhost (ovpn-8-19.pek2.redhat.com [10.72.8.19]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4FC09492C3B; Sun, 28 Aug 2022 04:50:15 +0000 (UTC) From: Ming Lei To: Jens Axboe Cc: linux-doc@vger.kernel.org, linux-block@vger.kernel.org, Christoph Hellwig , Ming Lei , Jonathan Corbet , "Richard W . M . Jones" , ZiyangZhang , Stefan Hajnoczi , Xiaoguang Wang Subject: [PATCH] Docs: ublk: add ublk document Date: Sun, 28 Aug 2022 12:50:03 +0800 Message-Id: <20220828045003.537131-1-ming.lei@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.85 on 10.11.54.10 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org ublk document is missed when merging ublk driver, so add it now. Cc: Jonathan Corbet Cc: Richard W.M. Jones Cc: ZiyangZhang Cc: Stefan Hajnoczi Cc: Xiaoguang Wang Signed-off-by: Ming Lei --- Documentation/block/ublk.rst | 203 +++++++++++++++++++++++++++++++++++ 1 file changed, 203 insertions(+) create mode 100644 Documentation/block/ublk.rst 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 +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. + +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( +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 +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 +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 +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 + driver to allocate & expose /dev/ublkb*. Parameters set via + UBLK_CMD_SET_PARAMS are applied for creating /dev/ublkb*. + +- 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 + should use this IO buffer to communicate with ublk driver. But existed + 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 + 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 +--------------------------- + +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. + +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