Commit c2c1accd authored May 02, 2007 by Zachary Amsden Committed by Andi Kleen May 02, 2007

[PATCH] i386: pte clear optimization

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>

parent df3624aa

include/asm-i386/pgtable-2level.h

+5 −0

Original line number	Diff line number	Diff line
		@@ -36,6 +36,11 @@ static inline void native_set_pmd(pmd_t *pmdp, pmd_t pmd)
		#define pte_clear(mm,addr,xp) do { set_pte_at(mm, addr, xp, __pte(0)); } while (0)
		#define pmd_clear(xp) do { set_pmd(xp, __pmd(0)); } while (0)

		static inline void native_pte_clear(struct mm_struct mm, unsigned long addr, pte_t xp)
		{
		*xp = __pte(0);
		}

		static inline pte_t native_ptep_get_and_clear(pte_t *xp)
		{
		return __pte(xchg(&xp->pte_low, 0));

include/asm-i386/pgtable.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -344,7 +344,7 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm, unsigned long
		pte_t pte;
		if (full) {
		pte = *ptep;
		pte_clear(mm, addr, ptep);
		native_pte_clear(mm, addr, ptep);
		} else {
		pte = ptep_get_and_clear(mm, addr, ptep);
		}