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

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Date: Wed, 19 Jun 2024 03:34:46 -0700
From: srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 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>, 
 "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, 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.

Thanks,
Srinivas