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    SubjectRe: [PATCH] x86: Support always running TSC on Intel CPUs
    On Tue, Nov 18, 2008 at 11:05:42AM -0500, Ingo Molnar wrote:
    >
    > * 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


    A name like X86_FEATURE_CONSTANT_FREQ_TSC implies that
    the result (the TSC) is constant frequency, not the input.


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