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SubjectRe: [PATCH] x86/tsc: Don't use cpuid 0x16 leaf to determine cpu speed.
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.

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