[PATCH] KVM: pvclock_clocksource_read while-loop optimization

[Arjan Koers <0h61vkll2ly8@xutrox.com>]

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 bab3b9e..995d479 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -126,7 +126,7 @@ 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);
@@ -158,8 +158,9 @@ 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;
 }