Re: [PATCH v2 1/2] riscv: introduce RISCV_EFFICIENT_UNALIGNED_ACCESS

[Charlie Jenkins <charlie@rivosinc.com>]

On Sun, Dec 03, 2023 at 09:57:52PM +0800, Jisheng Zhang wrote:
> Some riscv implementations such as T-HEAD's C906, C908, C910 and C920
> support efficient unaligned access, for performance reason we want
> to enable HAVE_EFFICIENT_UNALIGNED_ACCESS on these platforms. To
> avoid performance regressions on other non efficient unaligned access
> platforms, HAVE_EFFICIENT_UNALIGNED_ACCESS can't be globally selected.
> 
> To solve this problem, runtime code patching based on the detected
> speed is a good solution. But that's not easy, it involves lots of
> work to modify vairous subsystems such as net, mm, lib and so on.
> This can be done step by step.
> 
> So let's take an easier solution: add support to efficient unaligned
> access and hide the support under NONPORTABLE.
> 
> Now let's introduce RISCV_EFFICIENT_UNALIGNED_ACCESS which depends on
> NONPORTABLE, if users know during config time that the kernel will be
> only run on those efficient unaligned access hw platforms, they can
> enable it. Obviously, generic unified kernel Image shouldn't enable it.
> 
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
>  arch/riscv/Kconfig | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 7f8aa25457ba..0a76209e9b02 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -654,6 +654,18 @@ config RISCV_MISALIGNED
>  	  load/store for both kernel and userspace. When disable, misaligned
>  	  accesses will generate SIGBUS in userspace and panic in kernel.
>  
> +config RISCV_EFFICIENT_UNALIGNED_ACCESS

There already exists hwprobe for this purpose. If kernel code wants to
leverage the efficient unaligned accesses of hardware, it can use static
keys. I have a patch that will set this static key if the hardware was
detected to have fast unaligned accesses:

https://lore.kernel.org/linux-riscv/20231117-optimize_checksum-v11-2-7d9d954fe361@rivosinc.com/

- Charlie

> +	bool "Use unaligned access for some functions"
> +	depends on NONPORTABLE
> +	select HAVE_EFFICIENT_UNALIGNED_ACCESS
> +	default n
> +	help
> +	  Say Y here if you want the kernel only run on hardware platforms which
> +	  support efficient unaligned access, then unaligned access will be used
> +	  in some functions for optimized performance.
> +
> +	  If unsure what to do here, say N.
> +
>  endmenu # "Platform type"
>  
>  menu "Kernel features"
> -- 
> 2.42.0
> 
> 
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