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    Subject[PATCH 4.4 034/190] x86/tsc: Provide tsc=unstable boot parameter
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Peter Zijlstra <peterz@infradead.org>


    [ 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>
    ---
    arch/x86/kernel/tsc.c | 2 ++
    1 file changed, 2 insertions(+)

    --- a/arch/x86/kernel/tsc.c
    +++ b/arch/x86/kernel/tsc.c
    @@ -365,6 +365,8 @@ 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;
    }


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