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Message-ID: <871qa2zog6.fsf@all.your.base.are.belong.to.us>
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
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