linux-kernel - Re: [PATCH] x86: Support always running TSC on Intel CPUs

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Message-ID: <20081118160542.GC8088@elte.hu>
Date:	Tue, 18 Nov 2008 17:05:42 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Joe Korty <joe.korty@...r.com>
Cc:	Venki Pallipadi <venkatesh.pallipadi@...el.com>,
	H Peter Anvin <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Support always running TSC on Intel CPUs


* Joe Korty <joe.korty@...r.com> wrote:

> On Tue, Nov 18, 2008 at 09:09:52AM +0100, Ingo Molnar wrote:
> > 
> > * Venki Pallipadi <venkatesh.pallipadi@...el.com> wrote:
> > 
> > > +	if (c->x86_power & (1 << 8)) {
> > >  		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
> > > +		set_cpu_cap(c, X86_FEATURE_NOSTOP_TSC);
> > > +	}
> > 
> > hm, the naming is a bit confusing. We now have 3 variants:
> > 
> >   X86_FEATURE_TSC
> >   X86_FEATURE_CONSTANT_TSC
> >   X86_FEATURE_NOSTOP_TSC
> > 
> > NOSTOP_TSC is basically what CONSTANT_TSC should have been to begin 
> > with ;-)
> > 
> > i'd suggest to rename it to this:
> > 
> >   X86_FEATURE_TSC
> >   X86_FEATURE_CONSTANT_FREQ_TSC
> >   X86_FEATURE_STABLE_TSC
> > 
> > ... with CONSTANT_FREQ_TSC not having any real role in the long run. 
> > (it's similarly problematic to a completely unstable TSC)
> > 
> > does this sound ok?
> 
> 
> To me, the new naming has the same head-scratching potential
> as the old....
> 
> How about:
> 
> 	X86_FEATURE_TSC
> 	X86_FEATURE_STABLE_TSC_OBSOLETE
> 	X86_FEATURE_STABLE_TSC

the _honest_ naming would be:

 	X86_FEATURE_TSC
 	X86_FEATURE_STABLE_TSC_BUT_NOT_ALWAYS
 	X86_FEATURE_STABLE_TSC

;-) 

what's head-scratching about X86_FEATURE_CONSTANT_FREQ_TSC? It's a 
limited TSC variant: it follows a reference frequency that does not 
change with cpufreq changes, but it can stop at a whim in C states. So 
it's not "stable" nor really "constant" in the everyday sense.

What is 'constant' about it is its reference frequency - hence 
X86_FEATURE_CONSTANT_FREQ_TSC.

	Ingo
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