Re: [PATCH] LoongArch: Make -mstrict-align be configurable

["Arnd Bergmann" <arnd@arndb.de>]

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