Re: [Qemu-devel] [PATCH v5 7/9] block: don't make snapshots for filters

[Kevin Wolf <kwolf@redhat.com>]

Am 16.11.2016 um 10:49 hat Pavel Dovgalyuk geschrieben:
> > From: Pavel Dovgalyuk [mailto:dovgaluk@ispras.ru]
> > > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > My command line was assuming a raw image. It looks like you're using a
> > > qcow (hopefully qcow2?) image. If so, then you need to include the qcow2
> > > driver:
> > >
> > > -drive driver=blkreplay,if=none,image.driver=qcow2,\
> > > image.file.driver=file,image.file.filename=testdisk.qcow,id=img-blkreplay
> > 
> > This doesn't work for some reason. Replay just hangs at some moment.
> > 
> > Maybe there exists some internal difference between command line with one or two -drive
> > options?
> 
> I've investigated this issue.
> This command line works ok:
>  -drive driver=blkreplay,if=none,image.driver=file,image.filename=testdisk.qcow,id=img-blkreplay 
>  -device ide-hd,drive=img-blkreplay
> 
> And this does not:
>  -drive
> driver=blkreplay,if=none,image.driver=qcow2,image.file.driver=file,image.file.filename=testdisk.qcow
> ,id=img-blkreplay
>  -device ide-hd,drive=img-blkreplay
> 
> QEMU hangs at some moment of replay.
> 
> I found that some dma requests do not pass through the blkreplay driver
> due to the following line in block-backend.c:
>     return bdrv_co_preadv(blk->root, offset, bytes, qiov, flags);
> 
> This line passes read request directly to qcow driver and blkreplay cannot
> process it to make deterministic.

How does that bypass blkreplay? blk->root is supposed to be the blkreply
node, do you see something different? If it were the qcow2 node, then I
would expect that no requests at all go through the blkreplay layer.

Kevin