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

[Kevin Wolf]

Am 25.09.2019 um 11:02 hat Pavel Dovgalyuk geschrieben:
> > From: Kevin Wolf [mailto:address@hidden]
> > Am 20.09.2019 um 09:25 hat Pavel Dovgalyuk geschrieben:
> > > > From: Kevin Wolf [mailto:address@hidden]
> > > > Am 19.09.2019 um 14:10 hat Pavel Dovgalyuk geschrieben:
> > > > > > From: Kevin Wolf [mailto:address@hidden]
> > > > > > 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?

The reason why I didn't reply immediately is because I don't see
anything wrong in the options you used.

Just to confirm, do you still get the same error when you use only the
first -blockdev option and no other options at all?


I've now tried out the options you gave, and it does fail for me, but
with a different error:

    qemu-system-x86_64: -blockdev driver=blkreplay,file=hd0,node-name=hd0-rr: 
Invalid parameter 'blkreplay'

This one is because the QAPI schema doesn't know blkreplay and should
easily be fixed by adding a blkreplay field to BlockdevOptions.


As soft freeze is coming closer, I'm considering taking this series as
it is (it's wrong in parts, but the old state is probably even more
wrong) and letting you fix up these checks on top. What do you think?

Kevin