x86/tsc: Provide 'tsc=unstable' boot parameter

Peter Zijlstra · Ingo Molnar · commit 8309f86cd41e · 2017-05-15T10:15:14.000+02:00
Since the clocksource watchdog will only detect broken TSC after the
fact, all TSC based clocks will likely have observed non-continuous
values before/when switching away from TSC.

Therefore only thing to fully avoid random clock movement when your
BIOS randomly mucks with TSC values from SMI handlers is reporting the
TSC as unstable at boot.

Signed-off-by: Peter Zijlstra (Intel) &lt;peterz@infradead.org&gt;
Cc: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
Cc: Mike Galbraith &lt;efault@gmx.de&gt;
Cc: Peter Zijlstra &lt;peterz@infradead.org&gt;
Cc: Thomas Gleixner &lt;tglx@linutronix.de&gt;
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar &lt;mingo@kernel.org&gt;
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
@@ -374,6 +374,8 @@ static int __init tsc_setup(char *str)
 		tsc_clocksource_reliable = 1;
 	if (!strncmp(str, "noirqtime", 9))
 		no_sched_irq_time = 1;
+	if (!strcmp(str, "unstable"))
+		mark_tsc_unstable("boot parameter");
 	return 1;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -374,6 +374,8 @@ static int __init tsc_setup(char *str)`
`374`	`374`	`tsc_clocksource_reliable = 1;`
`375`	`375`	`if (!strncmp(str, "noirqtime", 9))`
`376`	`376`	`no_sched_irq_time = 1;`
	`377`	`+ if (!strcmp(str, "unstable"))`
	`378`	`+ mark_tsc_unstable("boot parameter");`
`377`	`379`	`return 1;`
`378`	`380`	`}`
`379`	`381`