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