linux-kernel - Re: [tip: perf/core] x86/cpu: Add helper function to get the type of the current hybrid CPU

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Date:   Fri, 5 May 2023 09:42:36 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org
Cc:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>,
        Len Brown <len.brown@...el.com>, Borislav Petkov <bp@...e.de>,
        x86@...nel.org
Subject: Re: [tip: perf/core] x86/cpu: Add helper function to get the type of
 the current hybrid CPU

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,

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?