Re: [Qemu-devel] [PATCH] vhdx: Return true for bdrv_has_zero_init

[Jeff Cody <jcody@redhat.com>]

On Fri, Dec 05, 2014 at 12:49:22PM +0100, Kevin Wolf wrote:
> Am 05.12.2014 um 11:26 hat Kevin Wolf geschrieben:
> > Like for most other image formats, vhdx images read as all zero in qemu
> > after their creation (we're taking advantage from the fact that qemu has
> > just created the image, because PAYLOAD_BLOCK_NOT_PRESENT actually means
> > undefined rather than zeroed according to the spec).
> > 
> > This fixes that 'qemu-img convert' to vhdx fully populates the image.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> 
> Jeff, another thing that Max found while we looked at the spec is that
> the 1.0 spec defines PAYLOAD_BLOCK_UNMAPPED as 3, but we define it as 5.
> It appears that the 0.95 spec actually had it that way.
> 
> Should we handle both 3 and 5 as unmapped?
> 

Wow - you are right, I checked the 1.0 spec change history. Since one
of the valid actions is to read zeros for PAYLOAD_BLOCK_UNMAPPED, it
would seem safe to do that for a technically undefined state of '5',
especially since that will maintain backwards compatibility of
0.95-spec images.  So I think 'yes'.