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

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Date:	Mon, 20 Jun 2016 16:20:26 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Bin Gao <bin.gao@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, bin.gao@...el.com
Subject: Re: x86/tsc: Set X86_FEATURE_TSC_RELIABLE to skip refined calibration

On Fri, Jun 17, 2016 at 12:48 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 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.

So, I'm worried my context here is a bit too stale to be of much use.
Generally, yea, if we can get the TSC freq from the hardware
registers, that would be ideal, as there's too many cases where the
hardware we're calibrating off of has problems.

But I feel like there were some early edge cases where the MSR didn't
report the right values on some cpus? I may be remembering this wrong,
as its been a few years.  That's my main concern, if we start skipping
the calibration completely, but I'd trust the intel folks have a
better sense of the edge cases here then my poor memory.

thanks
-john