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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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