netdev - Re: [PATCH RESEND bpf-next v3 4/6] riscv, bpf: Add necessary Zbb instructions

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Date: Sat, 27 Jan 2024 18:16:41 +0100
From: Björn Töpel <bjorn@...nel.org>
To: Pu Lehui <pulehui@...weicloud.com>, bpf@...r.kernel.org,
 linux-riscv@...ts.infradead.org, netdev@...r.kernel.org
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
 <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, Martin KaFai
 Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, Yonghong Song
 <yhs@...com>, John Fastabend <john.fastabend@...il.com>, KP Singh
 <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo
 <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, Palmer Dabbelt
 <palmer@...belt.com>, Conor Dooley <conor@...nel.org>, Luke Nelson
 <luke.r.nels@...il.com>, Pu Lehui <pulehui@...wei.com>, Pu Lehui
 <pulehui@...weicloud.com>
Subject: Re: [PATCH RESEND bpf-next v3 4/6] riscv, bpf: Add necessary Zbb
 instructions

Pu Lehui <pulehui@...weicloud.com> writes:

> From: Pu Lehui <pulehui@...wei.com>
>
> Add necessary Zbb instructions introduced by [0] to reduce code size and
> improve performance of RV64 JIT. Meanwhile, a runtime deteted helper is
> added to check whether the CPU supports Zbb instructions.
>
> Link: https://github.com/riscv/riscv-bitmanip/releases/download/1.0.0/bitmanip-1.0.0-38-g865e7a7.pdf [0]
> Signed-off-by: Pu Lehui <pulehui@...wei.com>
> ---
>  arch/riscv/net/bpf_jit.h | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
>
> diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
> index e30501b46f8f..51f6d214086f 100644
> --- a/arch/riscv/net/bpf_jit.h
> +++ b/arch/riscv/net/bpf_jit.h
> @@ -18,6 +18,11 @@ static inline bool rvc_enabled(void)
>  	return IS_ENABLED(CONFIG_RISCV_ISA_C);
>  }
>  
> +static inline bool rvzbb_enabled(void)
> +{
> +	return IS_ENABLED(CONFIG_RISCV_ISA_ZBB) && riscv_has_extension_likely(RISCV_ISA_EXT_ZBB);

Hmm, I'm thinking about the IS_ENABLED(CONFIG_RISCV_ISA_ZBB) semantics
for a kernel JIT compiler.

IS_ENABLED(CONFIG_RISCV_ISA_ZBB) affects the kernel compiler flags.
Should it be enough to just have the run-time check? Should a kernel
built w/o Zbb be able to emit Zbb from the JIT?


Björn