linux-kernel - Re: x86/tsc: Set X86_FEATURE_TSC_RELIABLE to skip refined calibration

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Message-ID: <alpine.DEB.2.11.1606170946561.5839@nanos>
Date:	Fri, 17 Jun 2016 09:48:13 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Bin Gao <bin.gao@...ux.intel.com>
cc:	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
	bin.gao@...el.com, John Stultz <john.stultz@...aro.org>
Subject: Re: x86/tsc: Set X86_FEATURE_TSC_RELIABLE to skip refined
 calibration

On Thu, 16 Jun 2016, Bin Gao wrote:

> Unlike PIT based calibration which counts TSC cycles against another timer,
> MSR or CPUID method has no calibration - it simply multiplies the known
> frequency of a timer by a ratio. So TSC frequency computed by MSR or CPUID
> is the final frequency and doesn't need the refined calibration process.
> We used to use set_cpu_cap(&boot_cpu_data, X86_FEATURE_TSC_RELIABLE) but
> it actually doesn't skip refined calibration because the flag is cleared
> later in identify_cpu(). A cpu caps flag is not cleared only if it's set
> by setup_force_cpu_cap(). This patch sets the flag in tsc_msr.c and
> replaces set_cpu_cap() with setup_force_cpu_cap() in other files.

I'm not entirely sure that this is correct. At least I want to know John
Stultz's opinion on that.
 
Thanks,

	tglx