[Qemu-devel] Re: [PATCH 3/3] block: Fix the use of protocols in backing files

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Thu, Dec 02, 2010 at 03:31:42PM +0100, Kevin Wolf wrote:
> Am 30.11.2010 16:14, schrieb Stefan Hajnoczi:
> > diff --git a/block.c b/block.c
> > index 59b69e4..c9a6720 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -611,10 +611,17 @@ 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);
> > +        if (path_has_protocol(bs->backing_file)) {
> > +            back_drv = bdrv_find_protocol(bs->backing_file);
> > +            pstrcpy(backing_filename, sizeof(backing_filename),
> > +                    bs->backing_file);
> > +        } else {
> > +            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);
> > +            }
> > +        }
> 
> Now we ignore bs->backing_format when a protocol is used. We don't even
> allow using anything on top of that protocol any more, so for example
> qcow2 over sheepdog would be broken. Maybe a use case that you like
> better would be QED over http or something.
> 
> It might still not be a very common use case, but this doesn't look
> right to me. Probably only the pstrcpy/path_combine should be conditional.

True, I'll take a look at that.  This reminds me of the write-up that
Markus did on block driver trees and formats vs protocols.

Stefan