It's still easier to return the full size of the buffer from KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2). It makes the userspace code
a bit easier.
OK. For the "full size" returned to userspace, would you prefer to
directly use the value retrieved from guest CPUID(0xd), or get it
from guest_fpu (i.e. fpstate->user_size)? (retrieved from CPUID will
be the max size and should work fine as well)