linux-kernel - [ 096/167] [PATCH] xen: correctly check for pending events when restoring irq flags

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Date:	Wed, 09 May 2012 06:52:05 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, David Vrabel <david.vrabel@...rix.com>,
	Ian Campbell <ian.campbell@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: [ 096/167] [PATCH] xen: correctly check for pending events when restoring irq
 flags

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

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

commit 7eb7ce4d2e8991aff4ecb71a81949a907ca755ac upstream.

In xen_restore_fl_direct(), xen_force_evtchn_callback() was being
called even if no events were pending.  This resulted in (depending on
workload) about a 100 times as many xen_version hypercalls as
necessary.

Fix this by correcting the sense of the conditional jump.

This seems to give a significant performance benefit for some
workloads.

There is some subtle tricksy "..since the check here is trying to
check both pending and masked in a single cmpw, but I think this is
correct. It will call check_events now only when the combined
mask+pending word is 0x0001 (aka unmasked, pending)." (Ian)

Acked-by: Ian Campbell <ian.campbell@...rix.com>
Signed-off-by: David Vrabel <david.vrabel@...rix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/xen/xen-asm.S |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/xen/xen-asm.S b/arch/x86/xen/xen-asm.S
index 79d7362..3e45aa0 100644
--- a/arch/x86/xen/xen-asm.S
+++ b/arch/x86/xen/xen-asm.S
@@ -96,7 +96,7 @@ ENTRY(xen_restore_fl_direct)

 	/* check for unmasked and pending */
 	cmpw $0x0001, PER_CPU_VAR(xen_vcpu_info) + XEN_vcpu_info_pending
-	jz 1f
+	jnz 1f
 2:	call check_events
 1:
 ENDPATCH(xen_restore_fl_direct)
-- 
1.7.10

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