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

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Date:   Fri, 26 Aug 2016 12:14:58 +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 Fri, 26 Aug 2016, Thomas Gleixner wrote:
> On Thu, 25 Aug 2016, Bin Gao wrote:
> > On Wed, Aug 24, 2016 at 10:51:20AM +0200, Thomas Gleixner wrote:
> > > 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.
> > 
> > MSR is only available on Intel Atom SoCs. There is no such a multi-socket system.
> 
> Fair enough.

Second thoughts. We should seperate the calibration aspect from the reliablity
aspect.

If a MSR/CPUID readout provides reliable calibration then this does not tell
us about the reliablity (i.e. no watchdog required). So having two flags for
this - and sure you can set both on those SoCs is the proper solution.

Thanks,

	tglx