I think using fixed offsets is unwise, since there's already contention
for the same leaves. Making sure that each block of leaves (where a
block is 16, 256 or some other number of leaves) is self-describing via
ABI signatures is the only sane way to go. There's still the issue of
assigning ABI signatures to vendors, but that's 1) less of an issue, and
2) can be self-assigned with very low likelihood of collision. That way
a guest can scan that region of leaf space for ABI signatures it
understand, and can pick and choose among what it finds (but not mix and
match - that sounds like a course for disaster).
If we use such a scheme, we can 1) avoid any existing users of that
space, 2) cleanly delimit a hypervisor-agnostic ABI portion of the leaf
space, and 3) allow hypervisors to implement multiple ABIs at once.
