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

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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