RE: [for-4.2 PATCH 3/6] replay: update docs for record/replay with block devices

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Kevin Wolf [mailto:kwolf@redhat.com]
> Am 20.09.2019 um 09:25 hat Pavel Dovgalyuk geschrieben:
> > > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > Am 19.09.2019 um 14:10 hat Pavel Dovgalyuk geschrieben:
> > > > > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > > > diff --git a/block/block-backend.c b/block/block-backend.c
> > > > > index 1c605d5444..c57d3d9fdf 100644
> > > > > --- a/block/block-backend.c
> > > > > +++ b/block/block-backend.c
> > > > > @@ -17,6 +17,7 @@
> > > > >  #include "block/throttle-groups.h"
> > > > >  #include "hw/qdev-core.h"
> > > > >  #include "sysemu/blockdev.h"
> > > > > +#include "sysemu/replay.h"
> > > > >  #include "sysemu/runstate.h"
> > > > >  #include "qapi/error.h"
> > > > >  #include "qapi/qapi-events-block.h"
> > > > > @@ -808,6 +809,12 @@ void blk_remove_bs(BlockBackend *blk)
> > > > >  int blk_insert_bs(BlockBackend *blk, BlockDriverState *bs, Error **errp)
> > > > >  {
> > > > >      ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
> > > > > +
> > > > > +    if (replay_mode != REPLAY_MODE_NONE && bs->drv != &bdrv_blkreplay) {
> > > > > +        error_setg(errp, "Root node must be blkreplay");
> > > > > +        return -ENOTSUP;
> > > > > +    }
> > > >
> > > > I guess this is opposite direction - bs->drv is bdrv_file.
> > > > And we should check its parent.
> > >
> > > If bs->drv is bdrv_file, you want this to fail because only
> > > bdrv_blkreplay should be able to be attached to devices.
> >
> > There was a regular rr invocation (as described in docs).
> > And bs->drv always was a pointer to bdrv_file: for original image,
> > and for temporary snapshot.
> 
> Hm, what was the actual command line you used? I can see that you have a
> separate -drive for the qcow2 file, so I can see how you get an unused
> BlockBackend for the qcow2 node, but I don't see how it would be a file
> node.
> 
> Anyway, this leaves us two options: Either change the recommended
> command line to use -blockdev for the qcow2 file so that no BlockBackend
> is created for it (I think this might be preferable), or restrict the
> error to when the BlockBackend is used.

I started playing with -blockdev: added new blockdev for blkreplay and
constructed the following command line:

-blockdev driver=file,filename=disk.img,node-name=hd0
-blockdev driver=blkreplay,file=hd0,node-name=hd0-rr
-device virtio-blk-device,drive=hd0-rr

However, I get an error: "Could not open 'disk.img': Permission denied"
Everything works when I use this file in '-drive' parameter.
What am I doing wrong?

Pavel Dovgalyuk