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Date:   Wed, 12 Apr 2023 13:50:32 +0000 (UTC)
From:   Paul Walmsley <paul@booyaka.com>
To:     Evan Green <evan@rivosinc.com>
cc:     Palmer Dabbelt <palmer@rivosinc.com>, heiko@sntech.de,
        linux-doc@vger.kernel.org,
        Andrew Bresticker <abrestic@rivosinc.com>,
        Atish Patra <atishp@rivosinc.com>,
        Conor Dooley <conor.dooley@microchip.com>,
        Celeste Liu <coelacanthus@outlook.com>,
        Jisheng Zhang <jszhang@kernel.org>,
        linux-riscv@lists.infradead.org, Jonathan Corbet <corbet@lwn.net>,
        Andrew Jones <ajones@ventanamicro.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Ley Foon Tan <leyfoon.tan@starfivetech.com>,
        vineetg@rivosinc.com, Philipp Tomsich <philipp.tomsich@vrull.eu>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Heiko Stuebner <heiko.stuebner@vrull.eu>, slewis@rivosinc.com,
        Samuel Holland <samuel@sholland.org>,
        linux-kernel@vger.kernel.org, Conor Dooley <conor@kernel.org>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Guo Ren <guoren@kernel.org>
Subject: Re: [PATCH v6 4/6] RISC-V: hwprobe: Support probing of misaligned
 access performance
In-Reply-To: <20230407231103.2622178-5-evan@rivosinc.com>
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References: <20230407231103.2622178-1-evan@rivosinc.com> <20230407231103.2622178-5-evan@rivosinc.com>
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On Fri, 7 Apr 2023, Evan Green wrote:

> This allows userspace to select various routines to use based on the
> performance of misaligned access on the target hardware.
> 
> Rather than adding DT bindings, this change taps into the alternatives
> mechanism used to probe CPU errata. Add a new function pointer alongside
> the vendor-specific errata_patch_func() that probes for desirable errata
> (otherwise known as "features"). Unlike the errata_patch_func(), this
> function is called on each CPU as it comes up, so it can save
> feature information per-CPU.
> 
> The T-head C906 has fast unaligned access, both as defined by GCC [1],
> and in performing a basic benchmark, which determined that byte copies
> are >50% slower than a misaligned word copy of the same data size (source
> for this test at [2]):
> 
> bytecopy size f000 count 50000 offset 0 took 31664899 us
> wordcopy size f000 count 50000 offset 0 took 5180919 us
> wordcopy size f000 count 50000 offset 1 took 13416949 us
> 
> [1] https://github.com/gcc-mirror/gcc/blob/master/gcc/config/riscv/riscv.cc#L353
> [2] https://pastebin.com/EPXvDHSW
> 
> Co-developed-by: Palmer Dabbelt <palmer@rivosinc.com>
> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> Signed-off-by: Evan Green <evan@rivosinc.com>
> Reviewed-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
> Tested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>

Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>


- Paul