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i386: Prevent early access to TSC to avoid crash on TSCless systems

PATCH In review Thomas Gleixner 3 participants 4 messages Mar 24, 2007

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Thomas Gleixner

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#0 lore raw

PATCH 1/1 i386: Prevent early access to TSC to avoid crash on TSCless systems

commit f9690982b8c2f9a2c65acdc113e758ec356676a3 removed the check for cpu_khz from sched_clock(), which prevented early access to the TSC by non obvious magic.

This is harmless as long as the CPU has a TSC. On TSCless systems this results in an illegal instruction trap.

Replace tsc_disabled and tsc_unstable by tsc_enabled, which is only set when the tsc is available and not unstable.

Signed-off-by Thomas Gleixner <[email protected]>
arch/i386/kernel/tsc.c +7 -2
index 0e65f7a..6cb8f53 100644
@@ -18,6 +18,8 @@
 
 #include "mach_timer.h"
 
+static int tsc_enabled;
+
 /*
  * On some systems the TSC frequency does not
  * change with the cpu frequency. So we need

@@ -105,7 +107,7 @@ unsigned long long sched_clock(void)
 	/*
 	 * Fall back to jiffies if there's no TSC available:
 	 */
-	if (tsc_unstable || unlikely(tsc_disable))
+	if (unlikely(!tsc_enabled))
 		/* No locking but a rare wrong value is not a big deal: */
 		return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
 

@@ -283,6 +285,7 @@ void mark_tsc_unstable(void)
 {
 	if (!tsc_unstable) {
 		tsc_unstable = 1;
+		tsc_enabled = 0;
 		/* Can be called before registration */
 		if (clocksource_tsc.mult)
 			clocksource_change_rating(&clocksource_tsc, 0);

@@ -383,7 +386,9 @@ void __init tsc_init(void)
 	if (check_tsc_unstable()) {
 		clocksource_tsc.rating = 0;
 		clocksource_tsc.flags &= ~CLOCK_SOURCE_IS_CONTINUOUS;
-	}
+	} else
+		tsc_enabled = 1;
+
 	clocksource_register(&clocksource_tsc);
 
 	return;
Acked-by on v1 1/1
#1 lore raw
Quoted 11 lines · Thomas Gleixner

commit f9690982b8c2f9a2c65acdc113e758ec356676a3 removed the check for cpu_khz from sched_clock(), which prevented early access to the TSC by non obvious magic.

This is harmless as long as the CPU has a TSC. On TSCless systems this results in an illegal instruction trap.

Replace tsc_disabled and tsc_unstable by tsc_enabled, which is only set when the tsc is available and not unstable.

Signed-off-by Thomas Gleixner <[email protected]>

oops, indeed! I think this should also resolve one of the bugs (TSC-less Cyrix?) that were reported against -mm some time ago.

Acked-by Ingo Molnar <[email protected]>

Ingo

#3 lore raw
Quoted 4 lines · Guillaume Chazarain
Quoted 1 line

+static int tsc_enabled;

So, now we have tsc_disable, tsc_enabled and tsc_unstable. I can understand the latter, but this lacks orthogonality IMHO.

tsc_disable should be renamed to tsc_disable_override or so, to signal that it's only the mirror of the 'notsc' flag the user passed in over the boot line.

Ingo