From mboxrd@z Thu Jan  1 00:00:00 1970
From: Arjan Koers <0h61vkll2ly8@xutrox.com>
Subject: [PATCH] KVM: pvclock_clocksource_read while-loop optimization
Date: Sat, 01 Jan 2011 21:51:03 +0100
Message-ID: <4D1F93B7.4080707@xutrox.com>
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Cc: kvm@vger.kernel.org, Glauber Costa <glommer@redhat.com>
To: Avi Kivity <avi@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>
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Reduce the number of while-loop iterations (from two to one in the most
common situation)

Signed-off-by: Arjan Koers <0h61vkll2ly8@xutrox.com>

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index 42eb330..8f52acb 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -93,7 +93,7 @@ cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
 	struct pvclock_shadow_time shadow;
 	unsigned version;
 	cycle_t ret, offset;
-	u64 last;
+	u64 last, save;

 	do {
 		version = pvclock_get_time_values(&shadow, src);
@@ -125,8 +125,9 @@ cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
 	do {
 		if (ret < last)
 			return last;
+		save = last;
 		last = atomic64_cmpxchg(&last_value, last, ret);
-	} while (unlikely(last != ret));
+	} while (unlikely(last != save));

 	return ret;
 }