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* [PATCH] xen/mm: do direct hypercall in xen_set_pte() if batching is unavailable
@ 2012-06-01 15:14 David Vrabel
  2012-06-05 16:07 ` Konrad Rzeszutek Wilk
  0 siblings, 1 reply; 6+ messages in thread
From: David Vrabel @ 2012-06-01 15:14 UTC (permalink / raw)
  To: xen-devel; +Cc: David Vrabel, Konrad Rzeszutek Wilk

From: David Vrabel <david.vrabel@citrix.com>

In xen_set_pte() if batching is unavailable (because the caller is in
an interrupt context such as handling a page fault) it would fall back
to using native_set_pte() and trapping and emulating the PTE write.

On 32-bit guests this requires two traps for each PTE write (one for
each dword of the PTE).  Instead, do one mmu_update hypercall
directly.

This significantly improves page fault performance in 32-bit PV
guests.

lmbench3 test  Before    After     Improvement
----------------------------------------------
lat_pagefault  3.18 us   2.32 us   27%
lat_proc fork  356 us    313.3 us  11%

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 arch/x86/xen/mmu.c |   16 ++++++++++++++--
 1 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index b8e2794..3bf5dfa 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -308,8 +308,20 @@ static bool xen_batched_set_pte(pte_t *ptep, pte_t pteval)
 
 static inline void __xen_set_pte(pte_t *ptep, pte_t pteval)
 {
-	if (!xen_batched_set_pte(ptep, pteval))
-		native_set_pte(ptep, pteval);
+	if (!xen_batched_set_pte(ptep, pteval)) {
+		/*
+		 * Could call native_set_pte() here and trap and
+		 * emulate the PTE write but with 32-bit guests this
+		 * needs two traps (one for each of the two 32-bit
+		 * words in the PTE) so do one hypercall directly
+		 * instead.
+		 */
+		struct mmu_update u;
+
+		u.ptr = virt_to_machine(ptep).maddr | MMU_NORMAL_PT_UPDATE;
+		u.val = pte_val_ma(pteval);
+		HYPERVISOR_mmu_update(&u, 1, NULL, DOMID_SELF);
+	}
 }
 
 static void xen_set_pte(pte_t *ptep, pte_t pteval)
-- 
1.7.2.5

^ permalink raw reply related	[flat|nested] 6+ messages in thread
end of thread, other threads:[~2012-06-13 16:10 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2012-06-01 15:14 [PATCH] xen/mm: do direct hypercall in xen_set_pte() if batching is unavailable David Vrabel
2012-06-05 16:07 ` Konrad Rzeszutek Wilk
2012-06-10 10:23   ` Konrad Rzeszutek Wilk
2012-06-11 10:23     ` David Vrabel
2012-06-11 12:29       ` Konrad Rzeszutek Wilk
2012-06-13 16:10         ` David Vrabel

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