linux-kernel - [PATCH 04/12] kvm/pvclock: add monotonicity check

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Date:	Wed, 14 Oct 2009 12:28:28 -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 04/12] kvm/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>
Cc: Avi Kivity <avi@...hat.com>
---
 arch/x86/kernel/kvmclock.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 223af43..46cb77f 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -77,9 +77,14 @@ static cycle_t kvm_clock_read(void)
 	return ret;
 }
 
+static struct clocksource kvm_clock;
+
 static cycle_t kvm_clock_get_cycles(struct clocksource *cs)
 {
-	return kvm_clock_read();
+	cycle_t ret = kvm_clock_read();
+
+	return ret >= kvm_clock.cycle_last ?
+		ret : kvm_clock.cycle_last;
 }
 
 /*
-- 
1.6.2.5

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