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 1AAACC4332F for ; Tue, 18 Oct 2022 13:18:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230340AbiJRNSC (ORCPT ); Tue, 18 Oct 2022 09:18:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54470 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230337AbiJRNSA (ORCPT ); Tue, 18 Oct 2022 09:18:00 -0400 Received: from mail-ed1-x533.google.com (mail-ed1-x533.google.com [IPv6:2a00:1450:4864:20::533]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5FAD7C8206 for ; Tue, 18 Oct 2022 06:17:58 -0700 (PDT) Received: by mail-ed1-x533.google.com with SMTP id l22so20387518edj.5 for ; Tue, 18 Oct 2022 06:17:58 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20210112.gappssmtp.com; s=20210112; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=17R+tJzGThcFb4auXXNy4eo0z69ICwDkXzx7bvwZ9Tk=; b=hkL+NhNCo4jp/y9ZYaHE1RaixEhUb6+a6HpzaGgOICwZgJfwzITlxEKgai+ErRE6P/ fYbK1JNf9K0e/t1zCDS03bZVpW9Ud0quz5vBfpUEWzFNLpZnav3oMNVWhX39UMJ/t3Mj FSOQQsKZeLNOlPkmsH8RLoUJdERbNc7Vh03y+SZBNw9+tfkRLEwrirKbxPpHyiMENmIc 5bjNOn7c7NJ/f8uTByNzssp+FddCb5knPBF2/zpRolrZn3DJTHvSSB/o1W1Vyj5d8ZOz Yd8m/VBygfO55Ze3OPE0SoOnMn38s4Oxq8sBczwItgAkskdIaLD9d8ergsI0zd2isath 01mQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=17R+tJzGThcFb4auXXNy4eo0z69ICwDkXzx7bvwZ9Tk=; b=xZO1kiLuzPFRFMPsVKqYCHf2NFFsvWLY3Q5g9VXgOjNQfSAQSOZ+2VfjCHLR9MQjfx TAfjYUCOEmJutTyb5TmGP9/a4I7iTtvDzlFdq3rDkF8nh3cOTDY8yaXicESO+5cEUp6m 7zipKsr66cgngN9rJ+9aQ88PxlU/1o1nC6khKCVlNLSO/Uxco7vhYFWf0fzoLy5NQlkL ABkN9hAVmCjWok67h/ykZIefUIxFm+We1P/RCHu7hcrTlHqLBXYdFgMDl3LaoJb5F8R0 ca90kzxeLwLq1MolX94fi4WJVDt42YfllIzbx+Fb6lLbxn2ieAH2o42PbQmSVPfpH01/ uUlw== X-Gm-Message-State: ACrzQf1VgMFwriZnlPEDEMBnwK1a24PSlAVWI9AG+3jFxTXZhGl5L0Ib hX2JpNisg/tV6yl0ndrGNnnYsewpFp1LIG9+sYBq X-Google-Smtp-Source: AMsMyM7GGszickWXLdxBAbKiYEICf2mOKwAaGF3Iec3KomfadkaT2u19nvXVm0Ga1s8PBdZrHl8nciM7NzTtGNncLRg= X-Received: by 2002:a50:eb81:0:b0:458:e40e:68d7 with SMTP id y1-20020a50eb81000000b00458e40e68d7mr2521569edr.131.1666099076659; Tue, 18 Oct 2022 06:17:56 -0700 (PDT) MIME-Version: 1.0 References: <50827796-af93-4af5-4121-dc13c31a67fc@linux.alibaba.com> In-Reply-To: From: Yongji Xie Date: Tue, 18 Oct 2022 21:17:45 +0800 Message-ID: Subject: Re: ublk-qcow2: ublk-qcow2 is available To: Ming Lei Cc: Stefan Hajnoczi , Ziyang Zhang , Stefan Hajnoczi , io-uring@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel , "Denis V. Lunev" , Xiaoguang Wang Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Tue, Oct 18, 2022 at 2:59 PM Ming Lei wrote: > > On Mon, Oct 17, 2022 at 07:11:59PM +0800, Yongji Xie wrote: > > On Fri, Oct 14, 2022 at 8:57 PM Ming Lei wrote: > > > > > > On Thu, Oct 13, 2022 at 02:48:04PM +0800, Yongji Xie wrote: > > > > On Wed, Oct 12, 2022 at 10:22 PM Stefan Hajnoczi wrote: > > > > > > > > > > On Sat, 8 Oct 2022 at 04:43, Ziyang Zhang wrote: > > > > > > > > > > > > On 2022/10/5 12:18, Ming Lei wrote: > > > > > > > On Tue, Oct 04, 2022 at 09:53:32AM -0400, Stefan Hajnoczi wro= te: > > > > > > >> On Tue, 4 Oct 2022 at 05:44, Ming Lei wrote: > > > > > > >>> > > > > > > >>> On Mon, Oct 03, 2022 at 03:53:41PM -0400, Stefan Hajnoczi w= rote: > > > > > > >>>> On Fri, Sep 30, 2022 at 05:24:11PM +0800, Ming Lei wrote: > > > > > > >>>>> ublk-qcow2 is available now. > > > > > > >>>> > > > > > > >>>> Cool, thanks for sharing! > > > > > > >>>> > > > > > > >>>>> > > > > > > >>>>> So far it provides basic read/write function, and compres= sion and snapshot > > > > > > >>>>> aren't supported yet. The target/backend implementation i= s completely > > > > > > >>>>> based on io_uring, and share the same io_uring with ublk = IO command > > > > > > >>>>> handler, just like what ublk-loop does. > > > > > > >>>>> > > > > > > >>>>> Follows the main motivations of ublk-qcow2: > > > > > > >>>>> > > > > > > >>>>> - building one complicated target from scratch helps libu= blksrv APIs/functions > > > > > > >>>>> become mature/stable more quickly, since qcow2 is compl= icated and needs more > > > > > > >>>>> requirement from libublksrv compared with other simple = ones(loop, null) > > > > > > >>>>> > > > > > > >>>>> - there are several attempts of implementing qcow2 driver= in kernel, such as > > > > > > >>>>> ``qloop`` [2], ``dm-qcow2`` [3] and ``in kernel qcow2(r= o)`` [4], so ublk-qcow2 > > > > > > >>>>> might useful be for covering requirement in this field > > > > > > >>>>> > > > > > > >>>>> - performance comparison with qemu-nbd, and it was my 1st= thought to evaluate > > > > > > >>>>> performance of ublk/io_uring backend by writing one ubl= k-qcow2 since ublksrv > > > > > > >>>>> is started > > > > > > >>>>> > > > > > > >>>>> - help to abstract common building block or design patter= n for writing new ublk > > > > > > >>>>> target/backend > > > > > > >>>>> > > > > > > >>>>> So far it basically passes xfstest(XFS) test by using ubl= k-qcow2 block > > > > > > >>>>> device as TEST_DEV, and kernel building workload is verif= ied too. Also > > > > > > >>>>> soft update approach is applied in meta flushing, and met= a data > > > > > > >>>>> integrity is guaranteed, 'make test T=3Dqcow2/040' covers= this kind of > > > > > > >>>>> test, and only cluster leak is reported during this test. > > > > > > >>>>> > > > > > > >>>>> The performance data looks much better compared with qemu= -nbd, see > > > > > > >>>>> details in commit log[1], README[5] and STATUS[6]. And th= e test covers both > > > > > > >>>>> empty image and pre-allocated image, for example of pre-a= llocated qcow2 > > > > > > >>>>> image(8GB): > > > > > > >>>>> > > > > > > >>>>> - qemu-nbd (make test T=3Dqcow2/002) > > > > > > >>>> > > > > > > >>>> Single queue? > > > > > > >>> > > > > > > >>> Yeah. > > > > > > >>> > > > > > > >>>> > > > > > > >>>>> randwrite(4k): jobs 1, iops 24605 > > > > > > >>>>> randread(4k): jobs 1, iops 30938 > > > > > > >>>>> randrw(4k): jobs 1, iops read 13981 write 14001 > > > > > > >>>>> rw(512k): jobs 1, iops read 724 write 728 > > > > > > >>>> > > > > > > >>>> Please try qemu-storage-daemon's VDUSE export type as well= . The > > > > > > >>>> command-line should be similar to this: > > > > > > >>>> > > > > > > >>>> # modprobe virtio_vdpa # attaches vDPA devices to host k= ernel > > > > > > >>> > > > > > > >>> Not found virtio_vdpa module even though I enabled all the = following > > > > > > >>> options: > > > > > > >>> > > > > > > >>> --- vDPA drivers > > > > > > >>> vDPA device simulator core > > > > > > >>> vDPA simulator for networking device > > > > > > >>> vDPA simulator for block device > > > > > > >>> VDUSE (vDPA Device in Userspace) support > > > > > > >>> Intel IFC VF vDPA driver > > > > > > >>> Virtio PCI bridge vDPA driver > > > > > > >>> vDPA driver for Alibaba ENI > > > > > > >>> > > > > > > >>> BTW, my test environment is VM and the shared data is done = in VM too, and > > > > > > >>> can virtio_vdpa be used inside VM? > > > > > > >> > > > > > > >> I hope Xie Yongji can help explain how to benchmark VDUSE. > > > > > > >> > > > > > > >> virtio_vdpa is available inside guests too. Please check tha= t > > > > > > >> VIRTIO_VDPA ("vDPA driver for virtio devices") is enabled in= "Virtio > > > > > > >> drivers" menu. > > > > > > >> > > > > > > >>> > > > > > > >>>> # modprobe vduse > > > > > > >>>> # qemu-storage-daemon \ > > > > > > >>>> --blockdev file,filename=3Dtest.qcow2,cache.direct= =3Dof|off,aio=3Dnative,node-name=3Dfile \ > > > > > > >>>> --blockdev qcow2,file=3Dfile,node-name=3Dqcow2 \ > > > > > > >>>> --object iothread,id=3Diothread0 \ > > > > > > >>>> --export vduse-blk,id=3Dvduse0,name=3Dvduse0,num-que= ues=3D$(nproc),node-name=3Dqcow2,writable=3Don,iothread=3Diothread0 > > > > > > >>>> # vdpa dev add name vduse0 mgmtdev vduse > > > > > > >>>> > > > > > > >>>> A virtio-blk device should appear and xfstests can be run = on it > > > > > > >>>> (typically /dev/vda unless you already have other virtio-b= lk devices). > > > > > > >>>> > > > > > > >>>> Afterwards you can destroy the device using: > > > > > > >>>> > > > > > > >>>> # vdpa dev del vduse0 > > > > > > >>>> > > > > > > >>>>> > > > > > > >>>>> - ublk-qcow2 (make test T=3Dqcow2/022) > > > > > > >>>> > > > > > > >>>> There are a lot of other factors not directly related to N= BD vs ublk. In > > > > > > >>>> order to get an apples-to-apples comparison with qemu-* a = ublk export > > > > > > >>>> type is needed in qemu-storage-daemon. That way only the d= ifference is > > > > > > >>>> the ublk interface and the rest of the code path is identi= cal, making it > > > > > > >>>> possible to compare NBD, VDUSE, ublk, etc more precisely. > > > > > > >>> > > > > > > >>> Maybe not true. > > > > > > >>> > > > > > > >>> ublk-qcow2 uses io_uring to handle all backend IO(include m= eta IO) completely, > > > > > > >>> and so far single io_uring/pthread is for handling all qcow= 2 IOs and IO > > > > > > >>> command. > > > > > > >> > > > > > > >> qemu-nbd doesn't use io_uring to handle the backend IO, so w= e don't > > > > > > > > > > > > > > I tried to use it via --aio=3Dio_uring for setting up qemu-nb= d, but not succeed. > > > > > > > > > > > > > >> know whether the benchmark demonstrates that ublk is faster = than NBD, > > > > > > >> that the ublk-qcow2 implementation is faster than qemu-nbd's= qcow2, > > > > > > >> whether there are miscellaneous implementation differences b= etween > > > > > > >> ublk-qcow2 and qemu-nbd (like using the same io_uring contex= t for both > > > > > > >> ublk and backend IO), or something else. > > > > > > > > > > > > > > The theory shouldn't be too complicated: > > > > > > > > > > > > > > 1) io uring passthough(pt) communication is fast than socket,= and io command > > > > > > > is carried over io_uring pt commands, and should be fast than= virio > > > > > > > communication too. > > > > > > > > > > > > > > 2) io uring io handling is fast than libaio which is taken in= the > > > > > > > test on qemu-nbd, and all qcow2 backend io(include meta io) i= s handled > > > > > > > by io_uring. > > > > > > > > > > > > > > https://github.com/ming1/ubdsrv/blob/master/tests/common/qcow= 2_common > > > > > > > > > > > > > > 3) ublk uses one single io_uring to handle all io commands an= d qcow2 > > > > > > > backend IOs, so batching handling is common, and it is easy t= o see > > > > > > > dozens of IOs/io commands handled in single syscall, or even = more. > > > > > > > > > > > > > >> > > > > > > >> I'm suggesting measuring changes to just 1 variable at a tim= e. > > > > > > >> Otherwise it's hard to reach a conclusion about the root cau= se of the > > > > > > >> performance difference. Let's learn why ublk-qcow2 performs = well. > > > > > > > > > > > > > > Turns out the latest Fedora 37-beta doesn't support vdpa yet,= so I built > > > > > > > qemu from the latest github tree, and finally it starts to wo= rk. And test kernel > > > > > > > is v6.0 release. > > > > > > > > > > > > > > Follows the test result, and all three devices are setup as s= ingle > > > > > > > queue, and all tests are run in single job, still done in one= VM, and > > > > > > > the test images are stored on XFS/virito-scsi backed SSD. > > > > > > > > > > > > > > The 1st group tests all three block device which is backed by= empty > > > > > > > qcow2 image. > > > > > > > > > > > > > > The 2nd group tests all the three block devices backed by pre= -allocated > > > > > > > qcow2 image. > > > > > > > > > > > > > > Except for big sequential IO(512K), there is still not small = gap between > > > > > > > vdpa-virtio-blk and ublk. > > > > > > > > > > > > > > 1. run fio on block device over empty qcow2 image > > > > > > > 1) qemu-nbd > > > > > > > running qcow2/001 > > > > > > > run perf test on empty qcow2 image via nbd > > > > > > > fio (nbd(/mnt/data/ublk_null_8G_nYbgF.qcow2), libaio, b= s 4k, dio, hw queues:1)... > > > > > > > randwrite: jobs 1, iops 8549 > > > > > > > randread: jobs 1, iops 34829 > > > > > > > randrw: jobs 1, iops read 11363 write 11333 > > > > > > > rw(512k): jobs 1, iops read 590 write 597 > > > > > > > > > > > > > > > > > > > > > 2) ublk-qcow2 > > > > > > > running qcow2/021 > > > > > > > run perf test on empty qcow2 image via ublk > > > > > > > fio (ublk/qcow2( -f /mnt/data/ublk_null_8G_s761j.qcow2)= , libaio, bs 4k, dio, hw queues:1, uring_comp: 0, get_data: 0). > > > > > > > randwrite: jobs 1, iops 16086 > > > > > > > randread: jobs 1, iops 172720 > > > > > > > randrw: jobs 1, iops read 35760 write 35702 > > > > > > > rw(512k): jobs 1, iops read 1140 write 1149 > > > > > > > > > > > > > > 3) vdpa-virtio-blk > > > > > > > running debug/test_dev > > > > > > > run io test on specified device > > > > > > > fio (vdpa(/dev/vdc), libaio, bs 4k, dio, hw queues:1)..= . > > > > > > > randwrite: jobs 1, iops 8626 > > > > > > > randread: jobs 1, iops 126118 > > > > > > > randrw: jobs 1, iops read 17698 write 17665 > > > > > > > rw(512k): jobs 1, iops read 1023 write 1031 > > > > > > > > > > > > > > > > > > > > > 2. run fio on block device over pre-allocated qcow2 image > > > > > > > 1) qemu-nbd > > > > > > > running qcow2/002 > > > > > > > run perf test on pre-allocated qcow2 image via nbd > > > > > > > fio (nbd(/mnt/data/ublk_data_8G_sc0SB.qcow2), libaio, b= s 4k, dio, hw queues:1)... > > > > > > > randwrite: jobs 1, iops 21439 > > > > > > > randread: jobs 1, iops 30336 > > > > > > > randrw: jobs 1, iops read 11476 write 11449 > > > > > > > rw(512k): jobs 1, iops read 718 write 722 > > > > > > > > > > > > > > 2) ublk-qcow2 > > > > > > > running qcow2/022 > > > > > > > run perf test on pre-allocated qcow2 image via ublk > > > > > > > fio (ublk/qcow2( -f /mnt/data/ublk_data_8G_yZiaJ.qcow2)= , libaio, bs 4k, dio, hw queues:1, uring_comp: 0, get_data: 0). > > > > > > > randwrite: jobs 1, iops 98757 > > > > > > > randread: jobs 1, iops 110246 > > > > > > > randrw: jobs 1, iops read 47229 write 47161 > > > > > > > rw(512k): jobs 1, iops read 1416 write 1427 > > > > > > > > > > > > > > 3) vdpa-virtio-blk > > > > > > > running debug/test_dev > > > > > > > run io test on specified device > > > > > > > fio (vdpa(/dev/vdc), libaio, bs 4k, dio, hw queues:1)..= . > > > > > > > randwrite: jobs 1, iops 47317 > > > > > > > randread: jobs 1, iops 74092 > > > > > > > randrw: jobs 1, iops read 27196 write 27234 > > > > > > > rw(512k): jobs 1, iops read 1447 write 1458 > > > > > > > > > > > > > > > > > > > > > > > > > > Hi All, > > > > > > > > > > > > We are interested in VDUSE vs UBLK, too. And I have tested them= with nullblk backend. > > > > > > Let me share some results here. > > > > > > > > > > > > I setup UBLK with: > > > > > > ublk add -t loop -f /dev/nullb0 -d QUEUE_DEPTH -q NR_QUEUE > > > > > > > > > > > > I setup VDUSE with: > > > > > > qemu-storage-daemon \ > > > > > > --chardev socket,id=3Dcharmonitor,path=3D/tmp/qmp.sock,s= erver=3Don,wait=3Doff \ > > > > > > --monitor chardev=3Dcharmonitor \ > > > > > > --blockdev driver=3Dhost_device,cache.direct=3Don,filena= me=3D/dev/nullb0,node-name=3Ddisk0 \ > > > > > > --export vduse-blk,id=3Dtest,node-name=3Ddisk0,name=3Dvd= use_test,writable=3Don,num-queues=3DNR_QUEUE,queue-size=3DQUEUE_DEPTH > > > > > > > > > > > > Here QUEUE_DEPTH is 1, 32 or 128 and NR_QUEUE is 1 or 4. > > > > > > > > > > > > Note: > > > > > > (1) VDUSE requires QUEUE_DEPTH >=3D 2. I cannot setup QUEUE_DEP= TH to 1. > > > > > > (2) I use qemu 7.1.0-rc3. It supports vduse-blk. > > > > > > (3) I do not use ublk null target so that the test is fair. > > > > > > (4) I setup fio with direct=3D1, bs=3D4k. > > > > > > > > > > > > ------------------------------ > > > > > > 1 job 1 iodepth, lat=EF=BC=88usec) > > > > > > vduse ublk > > > > > > seq-read 22.55 11.15 > > > > > > rand-read 22.49 11.17 > > > > > > seq-write 25.67 10.25 > > > > > > rand-write 24.13 10.16 > > > > > > > > > > Thanks for sharing. Any idea what the bottlenecks are for vduse a= nd ublk? > > > > > > > > > > > > > I think one reason for the latency gap of sync I/O is that vduse us= es > > > > workqueue in the I/O completion path but ublk doesn't. > > > > > > > > And one bottleneck for the async I/O in vduse is that vduse will do > > > > memcpy inside the critical section of virtqueue's spinlock in the > > > > virtio-blk driver. That will hurt the performance heavily when > > > > virtio_queue_rq() and virtblk_done() run concurrently. And it can b= e > > > > mitigated by the advance DMA mapping feature [1] or irq binding > > > > support [2]. > > > > > > Hi Yongji, > > > > > > Yeah, that is the cost you paid for virtio. Wrt. userspace block devi= ce > > > or other sort of userspace devices, cmd completion is driven by > > > userspace, not sure if one such 'irq' is needed. > > > > I'm not sure, it can be an optional feature in the future if needed. > > > > > Even not sure if virtio > > > ring is one good choice for such use case, given io_uring has been pr= oved > > > as very efficient(should be better than virtio ring, IMO). > > > > > > > Since vduse is aimed at creating a generic userspace device framework, > > virtio should be the right way IMO. > > OK, it is the right way, but may not be the effective one. > Maybe, but I think we can try to optimize it. > > And with the vdpa framework, the > > userspace device can serve both virtual machines and containers. > > virtio is good for VM, but not sure it is good enough for other > cases. > > > > > Regarding the performance issue, actually I can't measure how much of > > the performance loss is due to the difference between virtio ring and > > iouring. But I think it should be very small. The main costs come from > > the two bottlenecks I mentioned before which could be mitigated in the > > future. > > Per my understanding, at least there are two places where virtio ring is > less efficient than io_uring: > I might have misunderstood what you mean by virtio ring before. My previous understanding of the virtio ring does not include the virtio-blk driver. > 1) io_uring uses standalone submission queue(SQ) and completion queue(CQ)= , > so no contention exists between submission and completion; but virtio que= ue > requires per-vq lock in both submission and completion. > Yes, this is the bottleneck of the virtio-blk driver, even in the VM case. We are also trying to optimize this lock. One way to mitigate it is making submission and completion happen in the same core. > 2) io_uring can use single system call of io_uring_enter() for both > submitting and completing, so one context switch is enough, together > with natural batch processing for both submission and completion, and > it is observed that dozens or more than one hundred of IOs can be > covered in single syscall; virtio requires one notification for submissio= n and > another one for completion, looks at least two context switch are require= d > for handling one IO(s). > I'm not sure I get your point here. Looks like vduse doesn't need any syscall in the submitting path. And in the completion path, we can also do some batch processing then handle several I/Os in one single syscall. > > > > > ublk uses io_uring pt cmd for handling both io submission and complet= ion, > > > turns out the extra latency can be pretty small. > > > > > > BTW, one un-related topic, I saw the following words in > > > Documentation/userspace-api/vduse.rst: > > > > > > ``` > > > Note that only virtio block device is supported by VDUSE framework no= w, > > > which can reduce security risks when the userspace process that imple= ments > > > the data path is run by an unprivileged user. > > > ``` > > > > > > But when I tried to start qemu-storage-daemon for creating vdpa-virti= o > > > block by nor unprivileged user, 'Permission denied' is still returned= , > > > can you explain a bit how to start such process by unprivileged user? > > > Or maybe I misunderstood the above words, please let me know. > > > > > > > Currently vduse should only allow privileged users by default. But > > sysadmin can change the permission of the vduse char device or pass > > the device fd to an unprivileged process IIUC. > > I appreciate if you may provide a bit detailed steps for the above? > For example: 1. A privileged process creates a vduse device named "test" via /dev/vduse/control. 2. The privileged process changes the permission of /dev/vduse/test. 3. An unprivileged process opens the /dev/vduse/test to handle the I/O. > BTW, I changed the privilege of /dev/vduse/control to normal user, but > qemu-storage-daemon still returns 'Permission denied'. And if the > char dev is /dev/vduse/vduse0N, which is created by qemu-storage-daemon, > so how to change user of qemu-storage-daemon to unprivileged after > /dev/vduse/vduse0N is created? > I think qemu-storage-daemon doesn't support unprivileged users in current implementation. To support that, one extra privileged process is needed for device creation. Thanks, Yongji