Re: [Qemu-devel] [PATCH] block: fix the use of protocols in backing files

[Daniel P. Berrange]

On Thu, Oct 28, 2010 at 11:49:58AM +0200, Kevin Wolf wrote:
> Am 28.10.2010 11:35, schrieb Daniel P. Berrange:
> > On Thu, Oct 28, 2010 at 09:30:09AM +0100, Stefan Hajnoczi wrote:
> >> On Wed, Oct 27, 2010 at 7:19 PM, Anthony Liguori <address@hidden> wrote:
> >>> Signed-off-by: Anthony Liguori <address@hidden>
> >>>
> >>> diff --git a/block.c b/block.c
> >>> index 1a965b2..00b6f21 100644
> >>> --- a/block.c
> >>> +++ b/block.c
> >>> @@ -603,10 +603,16 @@ int bdrv_open(BlockDriverState *bs, const char 
> >>> *filename, int flags,
> >>>         BlockDriver *back_drv = NULL;
> >>>
> >>>         bs->backing_hd = bdrv_new("");
> >>> -        path_combine(backing_filename, sizeof(backing_filename),
> >>> -                     filename, bs->backing_file);
> >>> -        if (bs->backing_format[0] != '\0')
> >>> -            back_drv = bdrv_find_format(bs->backing_format);
> >>> +        back_drv = bdrv_find_protocol(bs->backing_file);
> >>> +        if (!back_drv) {
> >>> +            path_combine(backing_filename, sizeof(backing_filename),
> >>> +                         filename, bs->backing_file);
> >>> +            if (bs->backing_format[0] != '\0')
> >>> +                back_drv = bdrv_find_format(bs->backing_format);
> >>> +        } else {
> >>> +            pstrcpy(backing_filename, sizeof(backing_filename),
> >>> +                    bs->backing_file);
> >>> +        }
> >>>
> >>>         /* backing files always opened read-only */
> >>>         back_flags =
> >>> --
> >>> 1.7.0.4
> >>
> >> I think this makes sense.
> >>
> >> Now it is possible to specify backing files that are relative to
> >> QEMU's current working directory using file:filename.  I don't see
> >> harm in this.
> > 
> > Shouldn't a backing file be treated as relative to the image file pointing
> > to it, rather than the QEMU working directory. eg so you can do
> > 
> >   # qemu-img create backing.img
> >   # qemu-img create -o backing_file=file:backing.img main.img
> >   
> > And have main.img be able to resolve backing.img in its same directory,
> > no matter what directory QEMU itself is executing from
> 
> The problem is that this wouldn't work in the general case. It's rather
> an exception that it makes sense for file: backing files with file:
> images. Consider this:
> 
>   # qemu-img create -o backing_file=nbd:foo:1234 /tmp/main.img
> 
> Without this patch, you'll end up with /tmp/nbd:foo:1234, which is
> probably not what you wanted. With a patch that would work for file: you
> would get a hardly better path nbd:/tmp/foo:1234

Since we know the protocol, there could be a per-protocol function used
for resolving the backing store URI relative to the master URI. That
would avoid needing to special case file: in the shared generic code.

Daniel
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