linux-kernel - Re: Re: [PATCH 1/2] x86/tsc: add X86_FEATURE_TSC_KNOWN

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Date:   Thu, 10 Nov 2016 14:51:47 -0800
From:   Bin Gao <bin.gao@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        "x86@...nel.org Peter Zijlstra" <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Bin Gao <bin.gao@...el.com>
Subject: Re: Re: [PATCH 1/2] x86/tsc: add X86_FEATURE_TSC_KNOWN_FREQ flag

> > --- a/arch/x86/kernel/tsc.c
> > +++ b/arch/x86/kernel/tsc.c
> > @@ -1283,10 +1283,10 @@ static int __init init_tsc_clocksource(void)
> >  		clocksource_tsc.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;
> >  
> >  	/*
> > -	 * Trust the results of the earlier calibration on systems
> > -	 * exporting a reliable TSC.
> > +	 * When TSC frequency is known (generally got by MSR or CPUID), we skip
> > +	 * the refined calibration and directly register it as a clocksource.
> >  	 */
> > -	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE)) {
> > +	if (boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
> 
> This causes a regression, because with only this patch applied the architectures which use the reliable flag for this today are not longer taking this path.
> 
> The proper thing to do here is to make this:
> 
> 	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE) ||
> 	    boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
> 
> and remove the RELIABLE flag ckeck after the existing users are converted.

I Will fix this in next revision.
 
> Thanks,
> 
> 	tglx
>