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Date:   Wed, 26 Feb 2020 17:54:07 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Prarit Bhargava <prarit@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Patrick Geary <patrickg@supermicro.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        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
Subject: Re: [PATCH] x86/tsc: Add kernel options to disable CPUID and MSR
 calibrations
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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?

As it stands, I find this text a little too encouraging.

>  	tsx=		[X86] Control Transactional Synchronization
>  			Extensions (TSX) feature in Intel processors that