linux-kernel - Re: [RFC] x86, tsc: Add kcmdline args for skipping tsc calibration sequences

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Date:   Fri, 13 Jul 2018 12:40:41 -0700
From:   Arjan van de Ven <arjan@...ux.intel.com>
To:     patrickg <patrickg@...ermicro.com>, len.brown@...el.com,
        linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, alek.du@...el.com, feng.tang@...el.com
Subject: Re: [RFC] x86, tsc: Add kcmdline args for skipping tsc calibration
 sequences

On 7/13/2018 12:19 PM, patrickg wrote:
> This RFC patch is intended to allow bypass CPUID, MSR and QuickPIT calibration methods should the user desire to.
> 
> The current ordering in ML x86 tsc is to calibrate in the order listed above; returning whenever there's a successful calibration.  However there are certain BIOS/HW Designs for overclocking that cause the TSC to change along with the max core clock; and simple 'trusting' calibration methodologies will lead to the TSC running 'faster' and eventually, TSC instability.
> 


that would be a real violation of the contract between cpu and OS: tsc is not supposed to change for the duration of the boot

> I only know that there's a use-case for me to want to be able to skip CPUID calibration, however I included args for skipping all the rest just so that all functionality is covered in the long run instead of just one use-case.

wouldn't it be better to start the detailed calibration with the value from CPUID instead; that way we also properly calibrate spread spectrum etc...

I thought we switched to that recently to be honest...