[libvirt] [Qemu-devel] hindsight on qcow2v3 format

[Stefan Hajnoczi]

On Mon, Dec 16, 2013 at 05:24:53PM -0700, Eric Blake wrote:
> Is it worth tweaking docs/specs/qcow2.txt to permanently change the
> incompatible_features field to be 4 bytes (76-79) and mandate that bytes
> 72-75 always be 0, as a conservative way to prevent other misbehavior of
> programs like libvirt 0.10.2 that have code paths that don't correctly
> validate for version 2 files?  And if we ever really did hit a situation
> of having 31 or more incompatible features, I could envision using bit
> 31 as an incompatible_feature witness that tells new enough qemu to look
> at some other offset for remaining feature bits (thankfully, the
> semantics of the incompatible_feature field mean that older qemu that
> honors version 3 formats will reject a file with incompatible_feature
> bit 31 set), so this is no real loss in the number of potential
> incompatible features being added.

I don't think qcow2 changes are necessary, just make a libvirt 0.10.x
stable release that verifies the qcow2 version.

Stefan