I didn't see much of this last sentence in the actual patch, which ismach-xen/head.S is simpler than kernel/head.S, partly because it doesn't contain the CPU identification and feature tests (cpuid is assumed, though ironically, we don't actually use it).
good, because I just don't see any reason to do that at all; if they
features are there anyway, why not preserve their detection, it's not
hurting and it means you need to be less different "just because"...