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

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Date: Wed, 19 Jun 2024 23:25:56 +0200
From: Brice Goglin <brice.goglin@...il.com>
To: srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>,
 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>,
 "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 12:34, srinivas pandruvada a écrit :
> On Tue, 2024-06-18 at 18:53 -0700, Pawan Gupta wrote:
>> 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?
> But you still have to look at frequency or caches as there are Low
> power E-cores which will have same type but different capabilities.

Good point. From this patch series, I understand that the current kernel 
side doesn't care about these different E-cores. However it might be 
good to expose them as different cpu-types (or better name) to userspace ?

Something like type 0 = P-core, 1 = normal E-core, 2 = low power E-core ?

Brice