linux-kernel - [PATCH] Fix non-TSC guest clocksource lockup

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Date:	Thu, 09 Aug 2007 20:52:35 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	lguest <lguest@...abs.org>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	tglx <tglx@...utronix.de>, ron minnich <rminnich@...il.com>
Subject: [PATCH] Fix non-TSC guest clocksource lockup

lguest uses a host-supplied wallclock-based clocksource when the TSC
is not reliable.  As this is already in nanoseconds, I naively used a
multiplier of 1 and a shift of 0.

But update_wall_time() in its infinite wisdom decides to adjust the
clock a little (where does it think it's getting a more accurate time
from?)

It will happily tweak the multiplier... to 0, then -1.

So the "fix" is to use a shift of 22 like everyone else, and a
multiplier of 1 << 22.

Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>

diff -r e715c5d0ca6d drivers/lguest/lguest.c
--- a/drivers/lguest/lguest.c	Thu Aug 09 15:08:26 2007 +1000
+++ b/drivers/lguest/lguest.c	Thu Aug 09 20:42:46 2007 +1000
@@ -691,7 +691,8 @@ static struct clocksource lguest_clock =
 	.rating		= 400,
 	.read		= lguest_clock_read,
 	.mask		= CLOCKSOURCE_MASK(64),
-	.mult		= 1,
+	.mult		= 1 << 22,
+	.shift		= 22,
 };
 
 /* The "scheduler clock" is just our real clock, adjusted to start at zero */
@@ -774,7 +775,6 @@ static void lguest_time_init(void)
 	 * way, the "rating" is initialized so high that it's always chosen
 	 * over any other clocksource. */
 	if (lguest_data.tsc_khz) {
-		lguest_clock.shift = 22;
 		lguest_clock.mult = clocksource_khz2mult(lguest_data.tsc_khz,
 							 lguest_clock.shift);
 		lguest_clock.flags = CLOCK_SOURCE_IS_CONTINUOUS;


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