linux-kernel - Re: [PATCH] riscv, bpf: Optimize zextw insn with Zba extension

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Message-ID: <20240510-essay-subwoofer-e055375ff1cb@spud>
Date: Fri, 10 May 2024 21:56:07 +0100
From: Conor Dooley <conor@...nel.org>
To: Xiao Wang <xiao.w.wang@...el.com>
Cc: paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu,
	luke.r.nels@...il.com, xi.wang@...il.com, bjorn@...nel.org,
	ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
	martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org,
	yonghong.song@...ux.dev, john.fastabend@...il.com,
	kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
	jolsa@...nel.org, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
	pulehui@...wei.com, haicheng.li@...el.com
Subject: Re: [PATCH] riscv, bpf: Optimize zextw insn with Zba extension

On Tue, May 07, 2024 at 06:45:28PM +0800, Xiao Wang wrote:
> The Zba extension provides add.uw insn which can be used to implement
> zext.w with rs2 set as ZERO.
> 
> Signed-off-by: Xiao Wang <xiao.w.wang@...el.com>
> ---
>  arch/riscv/Kconfig       | 19 +++++++++++++++++++
>  arch/riscv/net/bpf_jit.h | 18 ++++++++++++++++++
>  2 files changed, 37 insertions(+)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 6bec1bce6586..0679127cc0ea 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -586,6 +586,14 @@ config RISCV_ISA_V_PREEMPTIVE
>  	  preemption. Enabling this config will result in higher memory
>  	  consumption due to the allocation of per-task's kernel Vector context.
>  
> +config TOOLCHAIN_HAS_ZBA
> +	bool
> +	default y
> +	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zba)
> +	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zba)
> +	depends on LLD_VERSION >= 150000 || LD_VERSION >= 23900
> +	depends on AS_HAS_OPTION_ARCH
> +
>  config TOOLCHAIN_HAS_ZBB
>  	bool
>  	default y
> @@ -601,6 +609,17 @@ config TOOLCHAIN_HAS_VECTOR_CRYPTO
>  	def_bool $(as-instr, .option arch$(comma) +v$(comma) +zvkb)
>  	depends on AS_HAS_OPTION_ARCH
>  
> +config RISCV_ISA_ZBA
> +	bool "Zba extension support for bit manipulation instructions"
> +	depends on TOOLCHAIN_HAS_ZBA
> +	depends on RISCV_ALTERNATIVE
> +	default y
> +	help
> +	   Adds support to dynamically detect the presence of the ZBA
> +	   extension (address generation acceleration) and enable its usage.

Recently I sent some patches to reword other extensions' help text,
because the "add support to dynamically detect" had confused people a
bit. Dynamic detection is done regardless of config options for Zba.
The wording I went with in my patch for Zbb was:
	   Add support for enabling optimisations in the kernel when the
	   Zbb extension is detected at boot.
Could you use something similar here in the opening sentence please?

Thanks,
Conor.

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