[PATCH] [27/34] i386: pte clear optimization

From: Andi Kleen
Date: Mon Apr 30 2007 - 11:51:46 EST

From: Zachary Amsden <zach@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andi Kleen <ak@xxxxxxx>


include/asm-i386/pgtable-2level.h | 5 +++++
include/asm-i386/pgtable.h | 2 +-
2 files changed, 6 insertions(+), 1 deletion(-)

Index: linux/include/asm-i386/pgtable-2level.h
--- linux.orig/include/asm-i386/pgtable-2level.h
+++ linux/include/asm-i386/pgtable-2level.h
@@ -36,6 +36,11 @@ static inline void native_set_pmd(pmd_t
#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));
Index: linux/include/asm-i386/pgtable.h
--- linux.orig/include/asm-i386/pgtable.h
+++ linux/include/asm-i386/pgtable.h
@@ -344,7 +344,7 @@ static inline pte_t ptep_get_and_clear_f
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);
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/