Feedback-ID: i56a14606:Fastmail
User-Agent: Cyrus-JMAP/3.9.0-alpha0-107-g82c3c54364-fm-20230131.002-g82c3c543
Mime-Version: 1.0
Message-Id: <ccf74ebd-ccc1-4de5-a425-dcde4ac39a8d@app.fastmail.com>
In-Reply-To: <20230202084238.2408516-1-chenhuacai@loongson.cn>
References: <20230202084238.2408516-1-chenhuacai@loongson.cn>
Date:   Thu, 02 Feb 2023 10:46:56 +0100
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Huacai Chen" <chenhuacai@loongson.cn>,
        "Huacai Chen" <chenhuacai@kernel.org>
Cc:     loongarch@lists.linux.dev, Linux-Arch <linux-arch@vger.kernel.org>,
        "Xuefeng Li" <lixuefeng@loongson.cn>, guoren <guoren@kernel.org>,
        "WANG Xuerui" <kernel@xen0n.name>,
        "Jiaxun Yang" <jiaxun.yang@flygoat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] LoongArch: Make -mstrict-align be configurable
Content-Type: text/plain
Precedence: bulk

On Thu, Feb 2, 2023, at 09:42, Huacai Chen wrote:
> Introduce Kconfig option ARCH_STRICT_ALIGN to make -mstrict-align be
> configurable.
>
> Not all LoongArch cores support h/w unaligned access, we can use the
> -mstrict-align build parameter to prevent unaligned accesses.
>
> This option is disabled by default to optimise for performance, but you
> can enabled it manually if you want to run kernel on systems without h/w
> unaligned access support.
>
> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

This feels like it's a way too low-level option, I would not expect
users to be able to answer this correctly.

What I would do instead is to have Kconfig options for specific
CPU implementations and derive the alignment requirements from
that.
 
> +config ARCH_STRICT_ALIGN
> +	bool "Enable -mstrict-align to prevent unaligned accesses"
> +	help
> +	  Not all LoongArch cores support h/w unaligned access, we can use
> +	  -mstrict-align build parameter to prevent unaligned accesses.
> +
> +	  This is disabled by default to optimise for performance, you can
> +	  enabled it manually if you want to run kernel on systems without
> +	  h/w unaligned access support.
> +


There is already a global CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
option, I think you should use that one instead of adding another
one. Setting HAVE_EFFICIENT_UNALIGNED_ACCESS for CPUs that can
do unaligned access will enable some important optimizations in
the network stack and a few other places.

    Arnd