linux-kernel - Re: [PATCH] arm64: cacheinfo: Report cache sets, ways, and line size

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Message-ID: <f63c2be5-50e4-4c47-8a56-9a570977a6cf@linux.dev>
Date: Mon, 12 May 2025 11:28:36 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Sudeep Holla <sudeep.holla@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
 linux-arm-kernel@...ts.infradead.org, Radu Rendec <rrendec@...hat.com>,
 Will Deacon <will@...nel.org>,
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
 Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: cacheinfo: Report cache sets, ways, and line size

On 5/10/25 03:04, Sudeep Holla wrote:
> On Fri, May 09, 2025 at 07:37:35PM -0400, Sean Anderson wrote:
>> Cache geometry is exposed through the Cache Size ID register. There is
>> one register for each cache, and they are selected through the Cache
>> Size Selection register. If FEAT_CCIDX is implemented, the layout of
>> CCSIDR changes to allow a larger number of sets and ways.
>> 
> 
> Please refer
> Commit a8d4636f96ad ("arm64: cacheinfo: Remove CCSIDR-based cache information probing")
> 

| The CCSIDR_EL1.{NumSets,Associativity,LineSize} fields are only for use
| in conjunction with set/way cache maintenance and are not guaranteed to
| represent the actual microarchitectural features of a design.
| 
| The architecture explicitly states:
| 
| | You cannot make any inference about the actual sizes of caches based
| | on these parameters.

However, on many cores (A53, A72, and surely others that I haven't
checked) these *do* expose the actual microarchitectural features of the
design. Maybe a whitelist would be suitable.

| Furthermore, CCSIDR_EL1.{WT,WB,RA,WA} have been removed retrospectively
| from ARMv8 and are now considered to be UNKNOWN.
| 
| Since the kernel doesn't make use of set/way cache maintenance and it is
| not possible for userspace to execute these instructions, we have no
| need for the CCSIDR information in the kernel.

Actually, these parameters are directly visible (and useful) to
userspace in the form of the cache size. Rather than make userspace
perform benchmarks, we can expose this information in a standard way.
There is of course [id]cache-size, but these properties are absent more
often than not:

$ git grep arm,cortex- 'arch/arm64/**.dtsi' | wc -l
1248
$ git grep d-cache-size 'arch/arm64/**.dtsi' | wc -l
320

--Sean