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Date:   Fri, 6 Dec 2019 11:39:42 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Krzysztof Piecuch <piecuch@protonmail.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
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        "malat@debian.org" <malat@debian.org>,
        "juri.lelli@redhat.com" <juri.lelli@redhat.com>
Subject: Re: [PATCH] x86/tsc: Don't use cpuid 0x16 leaf to determine cpu
 speed.
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On Thu, Dec 05, 2019 at 07:15:03PM +0000, Krzysztof Piecuch wrote:
> This patch corrects tsc drift on systems with changed base clock frequency
> (e.g. overclocking).
> 
> We can't use 0x16 cpu leaf as it's documented as "not reflecting actual
> values" and is supposed to be used only as a mean to determine "processor
> brand string and for determining the appropriate range to use when
> displaying processor information e.g. frequency history graphs".

What is the actual problem you're seeing? Because if CPUID.16h is used,
we don't set KNOWN_FREQ and will thus run tsc_refine_calibration_work()
(against HPET/PIT) later.

The CPUIS.16h value is only used as an initial guess.

> Signed-off-by: Krzysztof Piecuch <piecuch@protonmail.com>
> ---
>  arch/x86/kernel/tsc.c | 21 ++++-----------------
>  1 file changed, 4 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 7e322e2daaf5..fc9a000a814c 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -641,29 +641,16 @@ unsigned long native_calibrate_tsc(void)
>  			boot_cpu_data.x86_model == INTEL_FAM6_ATOM_GOLDMONT_D)
>  		crystal_khz = 25000;
> 
> +	if (crystal_khz == 0)
> +		return 0;
> +
>  	/*
>  	 * TSC frequency reported directly by CPUID is a "hardware reported"
>  	 * frequency and is the most accurate one so far we have. This
>  	 * is considered a known frequency.
>  	 */
> -	if (crystal_khz != 0)
> -		setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
> -
> -	/*
> -	 * Some Intel SoCs like Skylake and Kabylake don't report the crystal
> -	 * clock, but we can easily calculate it to a high degree of accuracy
> -	 * by considering the crystal ratio and the CPU speed.
> -	 */
> -	if (crystal_khz == 0 && boot_cpu_data.cpuid_level >= 0x16) {
> -		unsigned int eax_base_mhz, ebx, ecx, edx;
> -
> -		cpuid(0x16, &eax_base_mhz, &ebx, &ecx, &edx);
> -		crystal_khz = eax_base_mhz * 1000 *
> -			eax_denominator / ebx_numerator;
> -	}
> 
> -	if (crystal_khz == 0)
> -		return 0;
> +	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
> 
>  	/*
>  	 * For Atom SoCs TSC is the only reliable clocksource.

This completely screws over everything that doesn't have HPET/PIT and
doesn't have a useful CPUID.15h.

If you're on a system that has no HPET/PIT and also doesn't have a
useful CPUID.15h and CPUID.16h is wrong, then you're up a creek without
paddles.

So please, be more specific in your problem description.