Commit df17b1d9 authored Jun 15, 2008 by Mikael Pettersson Committed by Ingo Molnar Jun 19, 2008

x86, 32-bit: fix boot failure on TSC-less processors



Booting 2.6.26-rc6 on my 486 DX/4 fails with a "BUG: Int 6"
(invalid opcode) and a kernel halt immediately after the
kernel has been uncompressed. The BUG shows EIP pointing
to an rdtsc instruction in native_read_tsc(), invoked from
native_sched_clock().

(This error occurs so early that not even the serial console
can capture it.)

A bisection showed that this bug first occurs in 2.6.26-rc3-git7,
via commit 9ccc906c:

>x86: distangle user disabled TSC from unstable
>
>tsc_enabled is set to 0 from the command line switch "notsc" and from
>the mark_tsc_unstable code. Seperate those functionalities and replace
>tsc_enable with tsc_disable. This makes also the native_sched_clock()
>decision when to use TSC understandable.
>
>Preparatory patch to solve the sched_clock() issue on 32 bit.
>
>Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

The core reason for this bug is that native_sched_clock() gets
called before tsc_init().

Before the commit above, tsc_32.c used a "tsc_enabled" variable
which defaulted to 0 == disabled, and which only got enabled late
in tsc_init(). Thus early calls to native_sched_clock() would skip
the TSC and use jiffies instead.

After the commit above, tsc_32.c uses a "tsc_disabled" variable
which defaults to 0, meaning that the TSC is Ok to use. Early calls
to native_sched_clock() now erroneously try to use the TSC on
!cpu_has_tsc processors, leading to invalid opcode exceptions.

My proposed fix is to initialise tsc_disabled to a "soft disabled"
state distinct from the hard disabled state set up by the "notsc"
kernel option. This fixes the native_sched_clock() problem. It also
allows tsc_init() to be simplified: instead of setting tsc_disabled = 1
on every error return, we just set tsc_disabled = 0 once when all
checks have succeeded.

I've verified that this lets my 486 boot again. I've also verified
that a Core2 machine still uses the TSC as clocksource after the patch.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 75118a82

arch/x86/kernel/tsc_32.c

+8 −10

Original line number	Diff line number	Diff line
		@@ -14,7 +14,10 @@

		#include "mach_timer.h"

		static int tsc_disabled;
		/* native_sched_clock() is called before tsc_init(), so
		we must start with the TSC soft disabled to prevent
		erroneous rdtsc usage on !cpu_has_tsc processors */
		static int tsc_disabled = -1;

		/*
		* On some systems the TSC frequency does not
		@@ -402,25 +405,20 @@ void __init tsc_init(void)
		{
		int cpu;

		if (!cpu_has_tsc \|\| tsc_disabled) {
		/* Disable the TSC in case of !cpu_has_tsc */
		tsc_disabled = 1;
		if (!cpu_has_tsc \|\| tsc_disabled > 0)
		return;
		}

		cpu_khz = calculate_cpu_khz();
		tsc_khz = cpu_khz;

		if (!cpu_khz) {
		mark_tsc_unstable("could not calculate TSC khz");
		/*
		* We need to disable the TSC completely in this case
		* to prevent sched_clock() from using it.
		*/
		tsc_disabled = 1;
		return;
		}

		/* now allow native_sched_clock() to use rdtsc */
		tsc_disabled = 0;

		printk("Detected %lu.%03lu MHz processor.\n",
		(unsigned long)cpu_khz / 1000,
		(unsigned long)cpu_khz % 1000);

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