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

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Date: Tue, 18 Jun 2024 18:53:15 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Brice Goglin <brice.goglin@...il.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

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?