[PATCH 1/1] Switch to jiffies for native_sched_clock() when TSC warps

[Chase Douglas]

Some newer x86 processors (seen on core 2 duo, arrandale) warp their TSC
registers after a suspend. It is believed that a microcode fix may be a
solution, but for now we should work around the issue.

This change adds an upper bound on the difference between TSC readings. It
should be very generous (multiple year difference) so as to only catch TSC
warping which appears to generate timestamps many years in the future.
When a warp is found, usage of the TSC for timing is disabled. The
kernel falls back to using the jiffies counter, which is not as precise
but should be accurate.

Signed-off-by: Chase Douglas <chase.douglas at canonical.com>
---
 arch/x86/kernel/tsc.c |   15 +++++++++++++++
 1 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 597683a..3e2921d 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -43,7 +43,9 @@ static int tsc_clocksource_reliable;
 u64 native_sched_clock(void)
 {
 	u64 this_offset;
+	static u64 prev_offset;
 
+jiffies:
 	/*
 	 * Fall back to jiffies if there's no TSC available:
 	 * ( But note that we still use it if the TSC is marked
@@ -60,6 +62,19 @@ u64 native_sched_clock(void)
 	/* read the Time Stamp Counter: */
 	rdtscll(this_offset);
 
+	/*
+	 * if new time stamp is many years later, assume warping and disable
+	 * TSC usage:
+	 */
+	if (__cycles_2_ns(this_offset - prev_offset) > 0x100000000000000
+	    && prev_offset) {
+		printk(KERN_WARNING "TSC warped, using jiffies\n");
+		tsc_disabled = 1;
+		goto jiffies;
+	}
+
+	prev_offset = this_offset;
+
 	/* return the value in ns */
 	return __cycles_2_ns(this_offset);
 }
-- 
1.6.3.3