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    SubjectRe: [tip: perf/core] x86/cpu: Add helper function to get the type of the current hybrid CPU
    On Fri, May 05, 2023 at 09:42:36AM -0700, Dave Hansen wrote:
    > On 4/20/21 03:46, tip-bot2 for Ricardo Neri wrote:
    > > +#define X86_HYBRID_CPU_TYPE_ID_SHIFT 24
    > > +
    > > +/**
    > > + * get_this_hybrid_cpu_type() - Get the type of this hybrid CPU
    > > + *
    > > + * Returns the CPU type [31:24] (i.e., Atom or Core) of a CPU in
    > > + * a hybrid processor. If the processor is not hybrid, returns 0.
    > > + */
    > > +u8 get_this_hybrid_cpu_type(void)
    > > +{
    > > + if (!cpu_feature_enabled(X86_FEATURE_HYBRID_CPU))
    > > + return 0;
    > > +
    > > + return cpuid_eax(0x0000001a) >> X86_HYBRID_CPU_TYPE_ID_SHIFT;
    > > +}
    >
    > Hi Folks,

    Hi Dave,

    >
    > Sorry to dredge up an old thread. But, where does this information
    > about "If the processor is not hybrid, returns 0." come from?
    >
    > What is there to keep cpuid_eax(0x0000001a) from having 0x0 in those
    > bits? Seems to me like 0 is theoretically a valid hybrid CPU type. Right?

    My reasoning was that according to the Intel SDM the only valid values were
    0x20 and 0x40. 0 was meant to be an invalid value.

    I read the SDM again. It seems that cpuid_eax(0x0000001a) already returns
    0 when the leaf does not exist.

    Probably the check for X86_FEATURE_HYBRID_CPU is not needed.

    Still, callers need to check for a valid value, IMO.

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