Re: [Qemu-devel] [PATCH] block: don't add 'driver' to options when refering to backing via node name

[Peter Krempa <pkrempa@redhat.com>]

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On Tue, Oct 17, 2017 at 16:41:00 +0200, Kevin Wolf wrote:
> Am 12.10.2017 um 16:14 hat Peter Krempa geschrieben:
> > When refering to a backing file of an image via node name
> > bdrv_open_backing_file would add the 'driver' option to the option list
> > filling it with the backing format driver. This breaks construction of
> > the backing chain via -blockdev, as bdrv_open_inherit reports an error
> > if both 'reference' and 'options' are provided.
> > 
> > Signed-off-by: Peter Krempa <pkrempa@redhat.com>
> 
> If you don't mind, I'd add a specific example to the commit message:

I certainly don't mind. I was frustrated after some attempts to find
bugs in my code using it before looking into qemu so I forgot to add
the info. (Also it would take me some time to figure out that I have the
backing format specified in the overlay image).

> 
> $ qemu-img create -f raw /tmp/backing.raw 64M
> $ qemu-img create -f qcow2 -F raw -b /tmp/backing.raw /tmp/test.qcow2
> $ qemu-system-x86_64 \
>   -blockdev driver=file,filename=/tmp/backing.raw,node-name=backing \
>   -blockdev driver=qcow2,file.driver=file,file.filename=/tmp/test.qcow2,node-name=root,backing=backing
> qemu-system-x86_64: -blockdev driver=qcow2,file.driver=file,file.filename=/tmp/test.qcow2,node-name=root,backing=backing: Could not open backing file: Cannot reference an existing block device with additional options or a new filename
> 
> > diff --git a/block.c b/block.c
> > index 46eb1728da..684cb018da 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -2245,7 +2245,8 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
> >          goto free_exit;
> >      }
> > 
> > -    if (bs->backing_format[0] != '\0' && !qdict_haskey(options, "driver")) {
> > +    if (!reference &&
> > +        bs->backing_format[0] != '\0' && !qdict_haskey(options, "driver")) {
> >          qdict_put_str(options, "driver", bs->backing_format);
> >      }
> 
> Looks good to me.

Thanks.

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