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SubjectRe: [PATCH] x86: Support always running TSC on Intel CPUs

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

> On Tue, Nov 18, 2008 at 09:09:52AM +0100, Ingo Molnar wrote:
> >
> > * Venki Pallipadi <venkatesh.pallipadi@intel.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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