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Re: [PATCH] xen/arm: scan CLIDR Ctype fields upwards when probing LLC
- To: Mykola Kvach <xakep.amatop@xxxxxxxxx>
- From: Luca Fancellu <Luca.Fancellu@xxxxxxx>
- Date: Fri, 15 May 2026 11:50:05 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Mykola Kvach <mykola_kvach@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <Bertrand.Marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- Delivery-date: Fri, 15 May 2026 11:51:31 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH] xen/arm: scan CLIDR Ctype fields upwards when probing LLC
Hi Mykola,
> On 4 May 2026, at 10:19, Mykola Kvach <xakep.amatop@xxxxxxxxx> 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>
> ---
> This patch follows the xen-devel discussion:
> https://lists.xenproject.org/archives/html/xen-devel/2026-01/msg00345.html
>
> In that thread, Michal noted that the reverse scan was a simplification
> rather than an intentional requirement, and that changing the
> implementation would be fine.
>
> Testing performed:
> - standalone synthetic CLIDR tests covered both regular and pathological
> Ctype sequences and showed that the forward scan ignores unified cache
> levels above the first Ctype == 0b000 while the reverse scan can pick
> them
> - Renesas H3ULCB booted with llc-coloring=on
> ---
The changes looks ok to me
Reviewed-by: Luca Fancellu <luca.fancellu@xxxxxxx>
Cheers,
Luca
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