I-sync in pmap_remove_mapping() if the old mapping had PG_EXEC, and
kick curcpu (IMB) or other CPUs (via an IPI) only if the pmap was
the kernel pmap or active on other CPUs (curcpu is handled in userret()).
- Use lazy I-sync everywhere, (hopefully) eliminating the last of the
I-sync issues for multiprocessor support.
- Eliminate some memory barriers added in a couple of previous revisions,
after some discussion on port-alpha/tech-smp.
Still some lazy I-sync optimization possibilites:
- pmap_changebit() does not need to I-sync when only write-protecting
a page.
- pmap_asn_alloc() may be able to cancel a pending lazy I-sync when a
new ASN is allocated. Need to double check against Green Book or
Brown Book.
c7b8439ec4