When exiting from an address space, no special hypervisor notification of page
table updates needs to occur; direct page table hypervisors, such as Xen,
switch to another address space first (init_mm) and unprotects the page tables
to avoid the cost of trapping to the hypervisor for each pte_clear.  Shadow
mode hypervisors, such as VMI and lhype don't need to do the extra work of
calling through paravirt-ops, and can just directly clear the page table
entries without notifiying the hypervisor, since all the page tables are about
to be freed.

So introduce native_pte_clear functions which bypass any paravirt-ops
notification.  This results in a significant performance win for VMI and
removes some indirect calls from zap_pte_range.

Note the 3-level paging already had a native_pte_clear function, thus
demanding argument conformance and extra args for the 2-level definition.
Signed-off-by: Zachary Amsden <zach@vmware.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>