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

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.