linux-kernel - Re: [PATCH v2] x86/tsc: Set X86_FEATURE_TSC

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Date:   Wed, 24 Aug 2016 10:51:20 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Bin Gao <bin.gao@...ux.intel.com>
cc:     Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org, bin.gao@...el.com
Subject: Re: [PATCH v2] x86/tsc: Set X86_FEATURE_TSC_RELIABLE to skip refined
 calibration

On Tue, 16 Aug 2016, Bin Gao wrote:
> On some newer Intel x86 processors/SoCs the TSC frequency can be directly
> calculated by factors read from specific MSR registers or from a cpuid
> leaf (0x15). TSC frequency calculated by native msr/cpuid is absolutely
> accurate so we should always skip calibrating TSC aginst another clock,
> e.g. PIT, HPET, etc. So we want to skip the refined calibration by setting
> the X86_FEATURE_TSC_RELIABLE flag. Existing code setting the flag by
> set_cpu_cap() doesn't work as the flag is cleared later in identify_cpu().
> A cpu caps flag is not cleared only if it's set by setup_force_cpu_cap().
> This patch converted set_cpu_cap() to setup_force_cpu_cap() to ensure
> refined calibration is skipped.
> 
> We had a test on Intel CherryTrail platform: the 24 hours time drift is
> 3.6 seconds if refined calibration was not skipped while the drift is less
> than 0.6 second when refined calibration was skipped.
> 
> Correctly setting the X86_FEATURE_TSC_RELIABLE flag also guarantees TSC is
> not monitored by timekeeping watchdog because on most of these system TSC
> is the only reliable clocksource. HPET, for instance, works but may not
> be reliable. So kernel may report a physically reliable TSC is not reliable
> just because a physically not reliable HPET is acting as timekeeping
> watchdog.

What about non SoC systems where the MSR is available, but we still see that
cross socket TSC wreckage? This change will prevent the watchdog from
detecting that.

Thanks,

	tglx