netdev - Re: [PATCH v7 6/8] x86: tsc: Always Running Timer (ART) correlated clocksource

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Date:	Mon, 22 Feb 2016 18:38:11 -0800
From:	"Christopher Hall" <christopher.s.hall@...el.com>
To:	tglx@...utronix.de, richardcochran@...il.com, mingo@...hat.com,
	john.stultz@...aro.org, hpa@...or.com, jeffrey.t.kirsher@...el.com,
	"Andy Lutomirski" <luto@...nel.org>,
	"Peter Zijlstra" <peterz@...radead.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	kevin.b.stanton@...el.com, kevin.j.clarke@...el.com
Subject: Re: [PATCH v7 6/8] x86: tsc: Always Running Timer (ART) correlated
 clocksource

On Thu, 18 Feb 2016 13:11:33 -0800, Andy Lutomirski <luto@...nel.org>  
wrote:
>> +#define X86_FEATURE_INVARIANT_TSC (7*32+4) /* Intel Invariant TSC */

This is removed. It was basically an alias for NONSTOP_TSC and not needed.

>
>> +/*
>> + * Convert ART to TSC given numerator/denominator found in detect_art()
>> + */
>> +struct system_counterval_t convert_art_to_tsc(cycle_t art)
>> +{
>> +	u64 tmp, res, rem;
>> +
>> +	rem = do_div(art, art_to_tsc_denominator);
>> +
>> +	res = art * art_to_tsc_numerator;
>> +	tmp = rem * art_to_tsc_numerator;
>> +
>> +	do_div(tmp, art_to_tsc_denominator);
>> +	res += tmp;
>> +
>> +	return (struct system_counterval_t) {.cs = art_related_clocksource,
>> +			.cycles = res};
>
> The SDM and the patch description both mention an offset "k".  Shouldn't  
> this code at least have a comment about how it deals with the k != 0  
> case?

I don't deal with the k != 0 case. I assume that IA32 TSC adjust MSR is 0  
because it's almost always a *bad idea* to change it. I've discussed this  
with a few other developers and there is some consensus agreeing. From an  
earlier related thread Peter Zijlstra asserts that TSC adjust "had
better" be 0.(http://lkml.iu.edu/hypermail/linux/kernel/1507.3/03734.html).

Do we really need to accommodate BIOS's that do this?

Chris