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

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Date:   Wed, 22 Nov 2023 11:40:09 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Huang Shijie <shijie@...amperecomputing.com>,
        catalin.marinas@....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 09:48:57AM +0000, Will Deacon wrote:
> 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.

I agree, and I'd also like to drop ARM64_HAS_NO_HW_PREFETCH.

Mark.