netdev - Re: [PATCH bpf-next 3/7] riscv, bpf: Support sign-extension mov insns

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Message-ID: <87pm3dlj80.fsf@all.your.base.are.belong.to.us>
Date: Wed, 23 Aug 2023 20:14:07 +0200
From: Björn Töpel <bjorn@...nel.org>
To: Pu Lehui <pulehui@...weicloud.com>, linux-riscv@...ts.infradead.org,
 bpf@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Yonghong Song <yhs@...com>, 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>, 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>, Xu Kuohai
 <xukuohai@...wei.com>, Puranjay Mohan <puranjay12@...il.com>, Pu Lehui
 <pulehui@...wei.com>, Pu Lehui <pulehui@...weicloud.com>
Subject: Re: [PATCH bpf-next 3/7] riscv, bpf: Support sign-extension mov insns

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

> From: Pu Lehui <pulehui@...wei.com>
>
> Add support sign-extension mov instructions for RV64.
>
> Signed-off-by: Pu Lehui <pulehui@...wei.com>
> ---
>  arch/riscv/net/bpf_jit_comp64.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index fd36cb17101a..d1497182cacf 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -1047,7 +1047,19 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>  			emit_zext_32(rd, ctx);
>  			break;
>  		}
> -		emit_mv(rd, rs, ctx);
> +		switch (insn->off) {
> +		case 0:
> +			emit_mv(rd, rs, ctx);
> +			break;
> +		case 8:
> +		case 16:
> +			emit_slli(rs, rs, 64 - insn->off, ctx);
> +			emit_srai(rd, rs, 64 - insn->off, ctx);

You're clobbering the source register (rs) here, which is correct.

(Side note: Maybe it's time to add Zbb support to the JIT soon! ;-))


Björn