linux-kernel - Re: [PATCH 0/4] arm64: an optimization for AmpereOne

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Message-ID: <20231122094857.GA2959@willie-the-truck>
Date:   Wed, 22 Nov 2023 09:48:57 +0000
From:   Will Deacon <will@...nel.org>
To:     Huang Shijie <shijie@...amperecomputing.com>
Cc:     catalin.marinas@....com, mark.rutland@....com,
        suzuki.poulose@....com, broonie@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        anshuman.khandual@....com, robh@...nel.org, oliver.upton@...ux.dev,
        maz@...nel.org, patches@...erecomputing.com
Subject: Re: [PATCH 0/4] arm64: an optimization for AmpereOne

On Wed, Nov 22, 2023 at 05:28:51PM +0800, Huang Shijie wrote:
> 0) Background:
>    We found that AmpereOne benefits from aggressive prefetches when
>    using 4K page size.

We tend to shy away from micro-architecture specific optimisations in
the arm64 kernel as they're pretty unmaintainable, hard to test properly,
generally lead to bloat and add additional obstacles to updating our
library routines.

Admittedly, we have something for Thunder-X1 in copy_page() (disguised
as ARM64_HAS_NO_HW_PREFETCH) but, frankly, that machine needed all the
help it could get and given where it is today I suspect we could drop
that code without any material consequences.

So I'd really prefer not to merge this; modern CPUs should do better at
copying data. It's copy_to_user(), not rocket science.

Will