linux-kernel - Re: [PATCH] cache: Workaround HiSilicon Taishan DC CVAU

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Date:   Mon, 13 Dec 2021 18:56:43 +0000
From:   Will Deacon <will@...nel.org>
To:     Weilong Chen <chenweilong@...wei.com>
Cc:     catalin.marinas@....com, corbet@....net,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH] cache: Workaround HiSilicon Taishan DC CVAU

On Fri, Nov 26, 2021 at 05:11:39PM +0800, Weilong Chen wrote:
> Taishan's L1/L2 cache is inclusive, and the data is consistent.
> Any change of L1 does not require DC operation to brush CL in L1 to L2.
> It's safe that don't clean data cache by address to point of unification.
> 
> Without IDC featrue, kernel needs to flush icache as well as dcache,
> causes performance degradation.
> 
> The flaw refers to V110/V200 variant 1.
> 
> Signed-off-by: Weilong Chen <chenweilong@...wei.com>
> ---
>  Documentation/arm64/silicon-errata.rst |  2 ++
>  arch/arm64/Kconfig                     | 11 +++++++++
>  arch/arm64/include/asm/cputype.h       |  2 ++
>  arch/arm64/kernel/cpu_errata.c         | 32 ++++++++++++++++++++++++++
>  arch/arm64/tools/cpucaps               |  1 +
>  5 files changed, 48 insertions(+)

Hmm. We don't usually apply optimisations for specific CPUs on arm64, simply
because the diversity of CPUs out there means it quickly becomes a
fragmented mess.

Is this patch purely a performance improvement? If so, please can you
provide some numbers in an attempt to justify it?

Thanks,

Will