The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: b50db7095fe002fa3e16605546cba66bf1b68a3e Gitweb: https://git.kernel.org/tip/b50db7095fe002fa3e16605546cba66bf1b68a3e Author: Feng Tang feng.tang@intel.com AuthorDate: Wed, 17 Nov 2021 10:37:51 +08:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Thu, 02 Dec 2021 00:40:36 +01:00

x86/tsc: Disable clocksource watchdog for TSC on qualified platorms

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

--- arch/x86/kernel/tsc.c | 28 ++++++++++++++++++++++++---- 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index 2e076a4..a698196 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -1180,6 +1180,12 @@ 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) @@ -1196,6 +1202,23 @@ static void __init check_system_tsc_reliable(void) #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(); }

/* @@ -1387,9 +1410,6 @@ static int __init init_tsc_clocksource(void) 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;

@@ -1527,7 +1547,7 @@ 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();