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:   Wed, 10 May 2023 11:33:40 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        "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 5/10/23 11:18, Ricardo Neri wrote:
>> 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.

That doesn't make any sense to me really.  Just because today's SDM
doesn't have a value doesn't mean that it becomes an invalid value tomorrow.

For instance, there's no model 0xEE today.  But that doesn't make it
*INVALID*, it just means there's not one defined *today*.  Today's
kernel shouldn't fall over if it runs on an model==0xEE system.

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

Right, but this isn't really relevant here either.  A CPU's APICID comes
out of a leaf that can be unsupported (not exist).  That doesn't make
APICID==0 invalid in any way.

> Probably the check for X86_FEATURE_HYBRID_CPU is not needed.
> 
> Still, callers need to check for a valid value, IMO.

Right.  But if they're just going to check the number that comes back
from this function, 0 can't represent an invalid value.