Re: [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification

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

On Tue, Oct 12, 2010 at 10:07:53AM +0200, Kevin Wolf wrote:
> Am 11.10.2010 19:14, schrieb Anthony Liguori:
> > On 10/11/2010 11:18 AM, Anthony Liguori wrote:
> >> On 10/11/2010 10:46 AM, Stefan Hajnoczi wrote:
> >>> On Mon, Oct 11, 2010 at 05:39:01PM +0200, Avi Kivity wrote:
> >>>>   On 10/11/2010 05:30 PM, Stefan Hajnoczi wrote:
> >>>>>>   It was discussed before, but I don't think we came to a 
> >>>>>> conclusion. Are
> >>>>>>   there any circumstances under which you don't want to set the
> >>>>>>   QED_CF_BACKING_FORMAT flag?
> >>>>> I suggest the following:
> >>>>>
> >>>>> QED_CF_BACKING_FORMAT_RAW = 0x1
> >>>>>
> >>>>> When set, the backing file is a raw image and should not be probed for
> >>>>> its file format.  The default (unset) means that the backing image 
> >>>>> file
> >>>>> format may be probed.
> >>>>>
> >>>>> Now the backing_fmt_{offset,size} are no longer necessary.
> >>>> Should it not be an incompatible option?  If the backing disk starts
> >>>> with a format magic, it will be probed by an older qemu,
> >>>> incorrectly.
> >>> Agreed, it should be a non-compat feature bit.
> >>
> >> If it's just raw or not raw, then I agree it should be non-compat.
> >>
> >> I think we just need a feature bit then that indicates that the 
> >> backing file is non-probeable which certainly simplifies the 
> >> implementation.
> >>
> >> QED_F_BACKING_FORMAT_NOPROBE maybe?
> > 
> > Er, thinking more, this is still a good idea but we still need 
> > QED_CF_BACKING_FORMAT because we specifically need to know when a 
> > protocol is specified.  Otherwise, we have no way of doing nbd as a 
> > backing file.
> 
> Well, the protocol is currently encoded in the file name, separated by a
> colon. Of course, we want to get rid of that, but we still don't know
> what we want instead. It's completely unrelated to the backing file
> format, though, it's about the format of the backing file name.

I agree with Kevin.  There's no need to have the ill-defined backing
format AFAICT.

Stefan