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Subject: Re: [PATCH] x86/tsc: Add kernel options to disable CPUID and MSR
 calibrations
To:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org,
        Patrick Geary <patrickg@supermicro.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Juergen Gross <jgross@suse.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Daniel Drake <drake@endlessm.com>,
        Michael Zhivich <mzhivich@akamai.com>,
        linux-doc@vger.kernel.org
References: <20200226164308.14468-1-prarit@redhat.com>
 <20200226165407.GB18400@hirez.programming.kicks-ass.net>
 <87k149nd4p.fsf@nanos.tec.linutronix.de>
From:   Prarit Bhargava <prarit@redhat.com>
Message-ID: <64e49d3f-ac4c-84ca-e663-92f87aae1362@redhat.com>
Date:   Thu, 27 Feb 2020 07:58:43 -0500
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On 2/26/20 6:27 PM, Thomas Gleixner wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
>> On Wed, Feb 26, 2020 at 11:43:08AM -0500, Prarit Bhargava wrote:
>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>> index dbc22d684627..0316aadfff08 100644
>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>> @@ -4942,7 +4942,7 @@
>>>  			See Documentation/admin-guide/mm/transhuge.rst
>>>  			for more details.
>>>  
>>> -	tsc=		Disable clocksource stability checks for TSC.
>>> +	tsc=option[,option...]	Various TSC options.
>>>  			Format: <string>
>>>  			[x86] reliable: mark tsc clocksource as reliable, this
>>>  			disables clocksource verification at runtime, as well
>>> @@ -4960,6 +4960,12 @@
>>>  			in situations with strict latency requirements (where
>>>  			interruptions from clocksource watchdog are not
>>>  			acceptable).
>>> +			[x86] no_cpuid_calibration: Disable the CPUID TSC
>>> +			calibration.  Used in situations where the CPUID
>>> +			TSC khz does not match the actual CPU TSC khz
>>> +			[x86] no_msr_calibration: Disable the MSR TSC
>>> +			calibration.  Used in situations where the MSR
>>> +			TSC khz does not match the actual CPU TSC khz.
>>
>> Do we want to mention that these situations are mostly broken firmware?
>> Also do mention that if you disable these you might not boot due to not
>> having a PIT/HPET at all?
> 
> Right. Same discussion as before.
> 
> Also why do we want no_cpuid_calibration and no_msr_calibration? How


> should Joe User figure out which one to use? This does not make
> sense. The point is that the BIOS/Firmware supplied value in system
> registers is bogus. So something like "skip_firmware_calibration" might
> be better suitable.


no_cpuid_calibration was required for Patrick's case where the CPU was
overclocked and therefore the CPUID khz value was invalid, but the MSR value is
good.  I had to skip both to get to the PIT calibration because I had broken FW.
 I don't see how a single skip_firmware_calibration covers these cases.

> 
> Aside of that this really wants to be combined with the ability to
> supply the actual frequency on the command line as I suggested in the
> other thread to cope with machines which do not expose PIT/HPET or have
> broken variants of them.

tglx, can you give a lore link to the thread?

Thanks,

P.
> 
> Thanks,
> 
>         tglx
> 
>