linux-kernel - [PATCH 05/12] xen/pvclock: add monotonicity check

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Date:	Wed, 14 Oct 2009 12:28:29 -0700
From:	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	Xen-devel <xen-devel@...ts.xensource.com>, kurt.hackel@...cle.com,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Chris Mason <chris.mason@...cle.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: [PATCH 05/12] xen/pvclock: add monotonicity check

Other tsc-based clocksources add a monotonicity test to make sure there's
no regression in the returned cycles.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
---
 arch/x86/xen/time.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 0a5aa44..00f06cc 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -213,9 +213,14 @@ cycle_t xen_clocksource_read(void)
 	return ret;
 }
 
+static struct clocksource xen_clocksource;
+
 static cycle_t xen_clocksource_get_cycles(struct clocksource *cs)
 {
-	return xen_clocksource_read();
+	cycle_t ret = xen_clocksource_read();
+
+	return ret >= xen_clocksource.cycle_last ?
+		ret : xen_clocksource.cycle_last;
 }
 
 static void xen_read_wallclock(struct timespec *ts)
-- 
1.6.2.5

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