Re: [PATCH v4 01/47] x86/tsc: Never re-calibrate TSC frequency if its exact timing is known

[Borislav Petkov]

On Fri, May 29, 2026 at 07:43:48AM -0700, Sean Christopherson wrote:
> Don't re-calibrate the TSC frequency if the TSC is known to run at a fixed
> frequency.  In practice, this is likely one big nop, as re-calibration is
> used only for SMP=n kernels, and only for hardware that is 20+ years old,
> i.e. is extremely unlikely to collide with TSC_KNOWN_FREQ.

Why do we care?

So what if it recalibrates once on UP?

Look where it is called - all old rust which no one uses anymore.

> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kernel/tsc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index c5110eb554bc..08cf6625d484 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -946,7 +946,8 @@ void recalibrate_cpu_khz(void)
>  		return;
>  
>  	cpu_khz = x86_platform.calibrate_cpu();
> -	tsc_khz = x86_platform.calibrate_tsc();
> +	if (!boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ))

cpu_feature_enabled() everywhere please.

-- 
Regards/Gruss,
    Boris.

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