linux-kernel - Re: [PATCH 0/9] Add CPU-type to topology

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Date: Wed, 19 Jun 2024 23:22:16 +0200
From: Brice Goglin <brice.goglin@...il.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, daniel.sneddon@...ux.intel.com, tony.luck@...el.com,
 linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
 linux-perf-users@...r.kernel.org, Josh Poimboeuf <jpoimboe@...nel.org>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
 "Liang, Kan" <kan.liang@...ux.intel.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH 0/9] Add CPU-type to topology

Le 19/06/2024 à 03:53, Pawan Gupta a écrit :
> On Tue, Jun 18, 2024 at 02:49:10PM +0200, Brice Goglin wrote:
>> Le 17/06/2024 à 11:11, Pawan Gupta a écrit :
>>> Hi,
>>>
>>> This series adds support for CPU-type (CPUID.1A.EAX[31-24] on Intel) to
>>> differentiate between hybrid variants P+E, P-only, E-only that share the
>>> same Family/Model/Stepping. One of the use case for CPU-type is the
>>> affected CPU table for CPU vulnerabilities, which can now use the CPU-type
>>> to filter the unaffected variants.
>>>
>>> * Patch 1 adds cpu-type to CPU topology structure and introduces
>>>     topology_cpu_type() to get the CPU-type.
>>>
>>> * Patch 2-4 replaces usages of get_this_hybrid_cpu_type() with
>>>     topology_cpu_type().
>>>
>>> * Patch 5-7 Updates CPU-matching infrastructure to use CPU-type.
>>>
>>> * Patch 8 cleans up the affected CPU list.
>>>
>>> * Patch 9 uses the CPU-type to exclude P-only parts from the RFDS affected
>>>     list.
>>
>> Hello
>>
>> Is there still a plan to expose this info in sysfs?
> Sure, if it helps userspace.
>
>> Userspace currently uses frequencies to guess which cores are E or P.
>> Intel sent some patches several years ago [1], but they got abandoned
>> nowhere as far as I know. There was also some discussion about using a
>> "capacity" field like ARM does, but IIRC Intel didn't like the idea in
>> the end.
> There can be many ways to expose this information in sysfs. Like this ...
>
>> [1] https://lkml.org/lkml/2020/10/2/1208
> ... exposes /sys/devices/system/cpu/types which, in hybrid parts, creates a
> subdirectory for each type of CPU. Each subdirectory contains a CPU list
> and a CPU map that user space can query.
>
> The other way is to expose the CPU-type in a file:
>
> 	/sys/devices/system/cpu/cpuN/type
>
> that could return the CPU-type of the CPU N. Is there a preference?

I'd vote for the former.

Brice