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SubjectRe: Re: [PATCH 2/2] x86: use KNOWN_FREQ and RELIABLE TSC flags on certain processors/SoCs
On Thu, 10 Nov 2016, Bin Gao wrote:

> On Fri, Nov 11, 2016 at 12:26:40AM +0100, Thomas Gleixner wrote:
> > On Thu, 10 Nov 2016, Bin Gao wrote:
> > > > > @@ -702,6 +702,15 @@ unsigned long native_calibrate_tsc(void)
> > > > > }
> > > > > }
> > > > >
> > > > > + setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
> > > >
> > > > I can understand the one below, but this one changes existing behaviour w/o explaining why this is correct and desired. If at all then this wants to be a seperate patch and not just mingled in your goldmont update.
> > >
> > > native_calibrate_tsc() implements determining TSC frequency via CPUID.
> > > The purpose to add X86_FEATURE_TSC_KNOWN_FREQ flag is exactly for this case:
> > > TSC frequency determined via CPUID or MSR are always correct and the whole
> > > calibration should be skipped.
> >
> > Did you actually verify that this is correct and does not introduce NTP
> > issues compared to the long term calibration on such platforms?
> >
> > We've been burnt before and myself and others wasted enough time already
> > debugging that crap.
>
> Yes, we had a 24 hours test before on one of the CPUID capable platforms.
> With PIT calibrated frequency, we got more than 3 seconds drift whereas
> with CPUID determined frequency we only got less than 0.5 second drift.

So all that want's to be mentioned in the changelog when you add this flag
to that CPUID code path.

> Another fact is that on MSR capable platforms, PIT/HPET is generally not
> available so calibration won't work at all.

I know and still it want's to be documented. If you had done that in the
first place I would not ask those questions at all.

Thanks,

tglx



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