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

[Sudeep Holla <sudeep.holla@arm.com>]

On Mon, May 12, 2025 at 11:28:36AM -0400, Sean Anderson wrote:
> 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.

Yes that is already present, which is DT or ACPI.

> 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
>

Just to be clear, I am fine with exposing to the userspace, but just
not reading those registers as stated in the commit message I shared
earlier.

Why can't it be done via DT/ACPI ?

-- 
Regards,
Sudeep