From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:33232) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aUAHU-000594-Fn for qemu-devel@nongnu.org; Fri, 12 Feb 2016 04:44:53 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aUAHQ-0005AO-EJ for qemu-devel@nongnu.org; Fri, 12 Feb 2016 04:44:52 -0500 Received: from mx1.redhat.com ([209.132.183.28]:49696) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aUAHQ-00059w-48 for qemu-devel@nongnu.org; Fri, 12 Feb 2016 04:44:48 -0500 Date: Fri, 12 Feb 2016 10:44:41 +0100 From: Kevin Wolf Message-ID: <20160212094441.GA4828@noname.redhat.com> References: <20160210114531.GA5474@noname.redhat.com> <000601d163fb$4cbcae70$e6360b50$@Dovgaluk@ispras.ru> <20160210122816.GB5474@noname.redhat.com> <000a01d16401$be4d31d0$3ae79570$@Dovgaluk@ispras.ru> <20160210132545.GC5474@noname.redhat.com> <000601d16492$3a67d0a0$af3771e0$@Dovgaluk@ispras.ru> <20160211094343.GA4419@noname.redhat.com> <001001d164bb$789e0960$69da1c20$@Dovgaluk@ispras.ru> <20160211121829.GB4419@noname.redhat.com> <000b01d16570$099e8020$1cdb8060$@Dovgaluk@ispras.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <000b01d16570$099e8020$1cdb8060$@Dovgaluk@ispras.ru> Subject: Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Pavel Dovgalyuk Cc: edgar.iglesias@xilinx.com, peter.maydell@linaro.org, igor.rubinov@gmail.com, mark.burton@greensocs.com, real@ispras.ru, hines@cert.org, qemu-devel@nongnu.org, maria.klimushenkova@ispras.ru, stefanha@redhat.com, pbonzini@redhat.com, batuzovk@ispras.ru, alex.bennee@linaro.org, fred.konrad@greensocs.com Am 12.02.2016 um 09:33 hat Pavel Dovgalyuk geschrieben: > > From: Kevin Wolf [mailto:kwolf@redhat.com] > > Am 11.02.2016 um 12:00 hat Pavel Dovgalyuk geschrieben: > > > > From: Kevin Wolf [mailto:kwolf@redhat.com] > > > > Am 11.02.2016 um 07:05 hat Pavel Dovgalyuk geschrieben: > > > > > > From: Kevin Wolf [mailto:kwolf@redhat.com] > > > > > > Am 10.02.2016 um 13:51 hat Pavel Dovgalyuk geschrieben: > > > > > > > However, I don't understand yet which layer do you offer as the candidate > > > > > > > for record/replay? What functions should be changed? > > > > > > > I would like to investigate this way, but I don't got it yet. > > > > > > > > > > > > At the core, I wouldn't change any existing function, but introduce a > > > > > > new block driver. You could copy raw_bsd.c for a start and then tweak > > > > > > it. Leave out functions that you don't want to support, and add the > > > > > > necessary magic to .bdrv_co_readv/writev. > > > > > > > > > > > > Something like this (can probably be generalised for more than just > > > > > > reads as the part after the bdrv_co_reads() call should be the same for > > > > > > reads, writes and any other request types): > > > > > > > > > > > > int blkreplay_co_readv() > > > > > > { > > > > > > BlockReplayState *s = bs->opaque; > > > > > > int reqid = s->reqid++; > > > > > > > > > > > > bdrv_co_readv(bs->file, ...); > > > > > > > > > > > > if (mode == record) { > > > > > > log(reqid, time); > > > > > > } else { > > > > > > assert(mode == replay); > > > > > > bool *done = req_replayed_list_get(reqid) > > > > > > if (done) { > > > > > > *done = true; > > > > > > } else { > > > > > > req_completed_list_insert(reqid, qemu_coroutine_self()); > > > > > > qemu_coroutine_yield(); > > > > > > } > > > > > > } > > > > > > } > > > > > > > > > > > > /* called by replay.c */ > > > > > > int blkreplay_run_event() > > > > > > { > > > > > > if (mode == replay) { > > > > > > co = req_completed_list_get(e.reqid); > > > > > > if (co) { > > > > > > qemu_coroutine_enter(co); > > > > > > } else { > > > > > > bool done = false; > > > > > > req_replayed_list_insert(reqid, &done); > > > > > > /* wait synchronously for completion */ > > > > > > while (!done) { > > > > > > aio_poll(); > > > > > > } > > > > > > } > > > > > > } > > > > > > } > > > > > > > > > > > > Where we could consider changing existing code is that it might be > > > > > > desirable to automatically put an instance of this block driver on top > > > > > > of every block device when record/replay is used. If we don't do that, > > > > > > you need to explicitly specify -drive driver=blkreplay,... > > > > > > > > > > As far, as I understand, all synchronous read/write request are also passed > > > > > through this coroutines layer. > > > > > > > > Yes, all read/write requests go through the same function internally, no > > > > matter which external interface was used. > > > > > > > > > It means that every disk access in replay phase should match the recording phase. > > > > > > > > Right. If I'm not mistaken, this was the fundamental requirement you > > > > have, so I wouldn't have suggested this otherwise. > > > > > > > > > Record/replay is intended to be used for debugging and analysis. > > > > > When execution is replayed, guest machine cannot notice analysis overhead. > > > > > Some analysis methods may include disk image reading. E.g., qemu-based > > > > > analysis framework DECAF uses sleuthkit for disk forensics ( > > > > https://github.com/sycurelab/DECAF ). > > > > > If similar framework will be used with replay, forensics disk access operations > > > > > won't work if we will record/replay the coroutines. > > > > > > > > Sorry, I'm not sure if I can follow. > > > > > > > > If such analysis software runs in the guest, it's not a replay any more > > > > and I completely fail to see what you're doing. > > > > > > > > If it's a qemu component independent from the guest, then my method > > > > gives you a clean way to bypass the replay driver that wouldn't be > > > > possible with yours. > > > > > > The second one. qemu may be extended with some components that > > > perform guest introspection. > > > > > > > If your plan was to record/replay only async requests and then use sync > > > > requests to bypass the record/replay, let me clearly state that this is > > > > the wrong approach: There are still guest devices which do synchronous > > > > I/O and need to be considered in the replay log, and you shouldn't > > > > prevent the analysis code from using AIO (in fact, using sync I/O in new > > > > code is very much frowned upon). > > > > > > Why do guest synchronous requests have to be recorded? > > > Aren't they completely deterministic? > > > > Good point. I think you're right in practice. In theory, with dataplane > > (i.e. when running the request in a separate thread) it could happen, > > but I guess that isn't very compatible with replay anyway - and at the > > first sight I couldn't see it performing synchronous requests either. > > > > > > I can explain in more detail what the block device structure looks like > > > > and how to access an image with and without record/replay, but first let > > > > me please know whether I guessed right what your problem is. Or if I > > > > missed your point, can you please describe in detail a case that > > > > wouldn't work? > > > > > > You have understood it correctly. > > > And what is the solution for bypassing one of the layers from component that > > > should not affect the replay? > > > > For this, you need to understand how block drivers are stacked in qemu. > > Each driver in the stack has a separate struct BlockDriverState, which > > can be used to access its data. You could hook up things like this: > > > > virtio-blk NBD server > > -------------- ------------ > > | | > > v | > > +------------+ | > > | blkreplay | | > > +------------+ | > > | | > > v | > > +------------+ | > > | qcow2 | <---------------+ > > +------------+ > > | > > v > > +------------+ > > | raw-posix | > > +------------+ > > | > > v > > -------------- > > filesystem > > > > As you see, what I've chosen for the external analysis interface is just > > an NBD server as this is the component that we already have today. You > > could hook up any other (new) code there; the important part is that it > > doesn't work on the BDS of the blkreplay driver, but directly on the BDS > > of the qcow2 driver. > > > > On the command line, it could look like this (this assumes that we don't > > add syntactic sugar that creates the blkreplay part automatically - we > > can always do that): > > > > -drive file=image.qcow2,if=none,id=img-direct > > -drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay > > -device virtio-blk-pci,drive=img-blkreplay > > Are there any hints for driver with these options? > I can't figure out how to create _open function for that. > blkdebug driver seems similar, but it receives image name directly, without referencing > the lower level. Actually, the 'image' option of blkdebug works like this. For example, I just tried this commandline and it works: -drive file=/tmp/test.qcow2,if=none,id=img -drive driver=blkdebug,image=img,if=virtio Just ignore the legacy part with 'x-image' and pass NULL as the filename to bdrv_open_child(). Another driver that could be useful as an example is quorum. Kevin