Re: [PATCH v2] xen/arm: scan CLIDR Ctype fields upwards when probing LLC

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 5/25/26 1:47 PM, Mykola Kvach wrote:
> From: Mykola Kvach <mykola_kvach@xxxxxxxx>
>
> get_llc_way_size() currently scans CLIDR_EL1 Ctype fields from the
> highest level downwards and stops at the first unified cache it finds.
>
> However, CLIDR_EL1 describes the cache hierarchy from Ctype1 upwards.
> Arm ARM DDI 0487J.a, D19.2.27 says that once software has seen a
> Ctype value of 0b000 while reading from Ctype1 upwards, no caches
> manageable by the architected set/way maintenance instructions exist at
> further-out levels, and the higher Ctype fields must be ignored.
>
> The current reverse scan can therefore select a unified cache level from
> a Ctype field above the first no-cache level. Such a field is not part of
> the architecturally described CLIDR/CCSIDR cache hierarchy and should not
> be used for selecting the CCSIDR level.
>
> Scan Ctype fields from L1 upwards, stop at the first no-cache level, and
> keep the outermost unified cache observed before that point.
>
> This preserves the result for regular cache hierarchies, while avoiding
> selection of an architecturally ignored Ctype field.
>
> Fixes: f4985fce6f0b ("xen/arm: add initial support for LLC coloring on arm64")
> Signed-off-by: Mykola Kvach <mykola_kvach@xxxxxxxx>
> Reviewed-by: Luca Fancellu <luca.fancellu@xxxxxxx>
> Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Thanks.

~ Oleksii