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    Subject[PATCH 5.4 60/70] x86/tsc: Disable clocksource watchdog for TSC on qualified platorms
    Date
    From: Feng Tang <feng.tang@intel.com>

    commit b50db7095fe002fa3e16605546cba66bf1b68a3e upstream.

    There are cases that the TSC clocksource is wrongly judged as unstable by
    the clocksource watchdog mechanism which tries to validate the TSC against
    HPET, PM_TIMER or jiffies. While there is hardly a general reliable way to
    check the validity of a watchdog, Thomas Gleixner proposed [1]:

    "I'm inclined to lift that requirement when the CPU has:

    1) X86_FEATURE_CONSTANT_TSC
    2) X86_FEATURE_NONSTOP_TSC
    3) X86_FEATURE_NONSTOP_TSC_S3
    4) X86_FEATURE_TSC_ADJUST
    5) At max. 4 sockets

    After two decades of horrors we're finally at a point where TSC seems
    to be halfway reliable and less abused by BIOS tinkerers. TSC_ADJUST
    was really key as we can now detect even small modifications reliably
    and the important point is that we can cure them as well (not pretty
    but better than all other options)."

    As feature #3 X86_FEATURE_NONSTOP_TSC_S3 only exists on several generations
    of Atom processorz, and is always coupled with X86_FEATURE_CONSTANT_TSC
    and X86_FEATURE_NONSTOP_TSC, skip checking it, and also be more defensive
    to use maximal 2 sockets.

    The check is done inside tsc_init() before registering 'tsc-early' and
    'tsc' clocksources, as there were cases that both of them had been
    wrongly judged as unreliable.

    For more background of tsc/watchdog, there is a good summary in [2]

    [tglx} Update vs. jiffies:

    On systems where the only remaining clocksource aside of TSC is jiffies
    there is no way to make this work because that creates a circular
    dependency. Jiffies accuracy depends on not missing a periodic timer
    interrupt, which is not guaranteed. That could be detected by TSC, but as
    TSC is not trusted this cannot be compensated. The consequence is a
    circulus vitiosus which results in shutting down TSC and falling back to
    the jiffies clocksource which is even more unreliable.

    [1]. https://lore.kernel.org/lkml/87eekfk8bd.fsf@nanos.tec.linutronix.de/
    [2]. https://lore.kernel.org/lkml/87a6pimt1f.ffs@nanos.tec.linutronix.de/

    [ tglx: Refine comment and amend changelog ]

    Fixes: 6e3cd95234dc ("x86/hpet: Use another crystalball to evaluate HPET usability")
    Suggested-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Feng Tang <feng.tang@intel.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: "Paul E. McKenney" <paulmck@kernel.org>
    Cc: stable@vger.kernel.org
    Link: https://lore.kernel.org/r/20211117023751.24190-2-feng.tang@intel.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/x86/kernel/tsc.c | 28 ++++++++++++++++++++++++----
    1 file changed, 24 insertions(+), 4 deletions(-)

    --- a/arch/x86/kernel/tsc.c
    +++ b/arch/x86/kernel/tsc.c
    @@ -1162,6 +1162,12 @@ void mark_tsc_unstable(char *reason)

    EXPORT_SYMBOL_GPL(mark_tsc_unstable);

    +static void __init tsc_disable_clocksource_watchdog(void)
    +{
    + clocksource_tsc_early.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
    + clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
    +}
    +
    static void __init check_system_tsc_reliable(void)
    {
    #if defined(CONFIG_MGEODEGX1) || defined(CONFIG_MGEODE_LX) || defined(CONFIG_X86_GENERIC)
    @@ -1178,6 +1184,23 @@ static void __init check_system_tsc_reli
    #endif
    if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
    tsc_clocksource_reliable = 1;
    +
    + /*
    + * Disable the clocksource watchdog when the system has:
    + * - TSC running at constant frequency
    + * - TSC which does not stop in C-States
    + * - the TSC_ADJUST register which allows to detect even minimal
    + * modifications
    + * - not more than two sockets. As the number of sockets cannot be
    + * evaluated at the early boot stage where this has to be
    + * invoked, check the number of online memory nodes as a
    + * fallback solution which is an reasonable estimate.
    + */
    + if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
    + boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
    + boot_cpu_has(X86_FEATURE_TSC_ADJUST) &&
    + nr_online_nodes <= 2)
    + tsc_disable_clocksource_watchdog();
    }

    /*
    @@ -1369,9 +1392,6 @@ static int __init init_tsc_clocksource(v
    if (tsc_unstable)
    goto unreg;

    - if (tsc_clocksource_reliable || no_tsc_watchdog)
    - clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
    -
    if (boot_cpu_has(X86_FEATURE_NONSTOP_TSC_S3))
    clocksource_tsc.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;

    @@ -1506,7 +1526,7 @@ void __init tsc_init(void)
    }

    if (tsc_clocksource_reliable || no_tsc_watchdog)
    - clocksource_tsc_early.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
    + tsc_disable_clocksource_watchdog();

    clocksource_register_khz(&clocksource_tsc_early, tsc_khz);
    detect_art();

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