linux-kernel - Re: [PATCH v3 5/7] RISC-V: hwprobe: Support probing of misaligned access performance

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Message-ID: <Y/05cBMyrYSiaNL5@spud>
Date:   Mon, 27 Feb 2023 23:14:56 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Evan Green <evan@...osinc.com>
Cc:     Palmer Dabbelt <palmer@...osinc.com>, heiko@...ech.de,
        slewis@...osinc.com, vineetg@...osinc.com,
        Albert Ou <aou@...s.berkeley.edu>,
        Andrew Bresticker <abrestic@...osinc.com>,
        Andrew Jones <ajones@...tanamicro.com>,
        Anup Patel <apatel@...tanamicro.com>,
        Atish Patra <atishp@...osinc.com>,
        Celeste Liu <coelacanthus@...look.com>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Guo Ren <guoren@...nel.org>,
        Jisheng Zhang <jszhang@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Ruizhe Pan <c141028@...il.com>,
        Sunil V L <sunilvl@...tanamicro.com>,
        Tsukasa OI <research_trasio@....a4lg.com>,
        Xianting Tian <xianting.tian@...ux.alibaba.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v3 5/7] RISC-V: hwprobe: Support probing of misaligned
 access performance

Hey Evan,

On Tue, Feb 21, 2023 at 11:08:56AM -0800, Evan Green wrote:
> This allows userspace to select various routines to use based on the
> performance of misaligned access on the target hardware.
> 
> Co-developed-by: Palmer Dabbelt <palmer@...osinc.com>
> Signed-off-by: Palmer Dabbelt <palmer@...osinc.com>
> Signed-off-by: Evan Green <evan@...osinc.com>
> 
> ---
> 
> Changes in v3:
>  - Have hwprobe_misaligned return int instead of long.
>  - Constify cpumask pointer in hwprobe_misaligned()
>  - Fix warnings in _PERF_O list documentation, use :c:macro:.
>  - Move include cpufeature.h to misaligned patch.
>  - Fix documentation mismatch for RISCV_HWPROBE_KEY_CPUPERF_0 (Conor)
>  - Use for_each_possible_cpu() instead of NR_CPUS (Conor)
>  - Break early in misaligned access iteration (Conor)
>  - Increase MISALIGNED_MASK from 2 bits to 3 for possible UNSUPPORTED future
>    value (Conor)

I'm not quite sure why we don't just go ahead and plumb this in already?
Whether the specs allow this or not, someone is going to end up doing
it (and it sounds like the specs now do allow it).
Is it wise to plug the hole in the syscall now, rather than leaving the
gap?

Otherwise, this looks fine, modulo Joe's comment about types.

Cheers,
Conor.


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