Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification

[Vladimir Sementsov-Ogievskiy]

On 19.01.2016 20:29, Kevin Wolf wrote:
> Am 19.01.2016 um 09:57 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > On 19.01.2016 00:16, Eric Blake wrote:
> > > preserving semantics of those extra_data bytes).  We
> > > have enough room for future extension, and that's good e
> > Ok, so, what should go to the spec? Current wording is ok? Just
> > delete "Type-specific":
> >
> > +
> > +        20 - 23:    extra_data_size
> > +                    Size of type-specific extra data.
> > +
> > +                    For now, as no extra data is defined, extra_data_size is
> > +                    reserved and must be zero.
> > +
> > +        variable:   Extra data for the bitmap.
> Please be explicit that if extra_data_size is non-zero, the bitmap must
> not be used (i.e. specify the incompatible-feature-bit-like behaviour).

It is not enough. If there are some unknown extra data, then just 
ignoring this bitmap may lead to its inconsistency. So, if it is 
non-zero, the whole image should not be written. (real 
incompatible-feature-bit behavior).

> Kevin


--
Best regards,
Vladimir
* now, @virtuozzo.com instead of @parallels.com. Sorry for this inconvenience.