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

From: Conor Dooley <conor@kernel.org>
To: Xiao Wang <xiao.w.wang@intel.com>
Cc: paul.walmsley@sifive.com, palmer@dabbelt.com,
	aou@eecs.berkeley.edu, luke.r.nels@gmail.com, xi.wang@gmail.com,
	bjorn@kernel.org, ast@kernel.org, daniel@iogearbox.net,
	andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com,
	song@kernel.org, yonghong.song@linux.dev,
	john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com,
	haoluo@google.com, jolsa@kernel.org,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	bpf@vger.kernel.org, pulehui@huawei.com, haicheng.li@intel.com
Subject: Re: [PATCH] riscv, bpf: Optimize zextw insn with Zba extension
Date: Fri, 10 May 2024 21:56:07 +0100	[thread overview]
Message-ID: <20240510-essay-subwoofer-e055375ff1cb@spud> (raw)
In-Reply-To: <20240507104528.435980-1-xiao.w.wang@intel.com>

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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@intel.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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