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

[ Upstream commit 8309f86cd41e8714526867177facf7a316d9be53 ]

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) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index d07a9390023e0194f05e313a899df6f5a6cf0629..bbfb03eccb7f16bb23da347fc526b99eb260b4a0 100644 (file)
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -366,6 +366,8 @@ 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;
 }