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Date: Thu, 28 Sep 2023 12:55:53 +0200
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 bpf-next v2 3/6] riscv, bpf: Simplify sext and zext
logics in branch instructions
Pu Lehui <pulehui@...weicloud.com> writes:
> From: Pu Lehui <pulehui@...wei.com>
>
> There are many extension helpers in the current branch instructions, and
> the implementation is a bit complicated. We simplify this logic through
> two simple extension helpers with alternate register.
>
> Signed-off-by: Pu Lehui <pulehui@...wei.com>
> ---
> arch/riscv/net/bpf_jit_comp64.c | 82 +++++++++++++--------------------
> 1 file changed, 31 insertions(+), 51 deletions(-)
>
> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index 4a649e195..0c6ffe11a 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -141,6 +141,19 @@ static bool in_auipc_jalr_range(s64 val)
> val < ((1L << 31) - (1L << 11));
> }
>
> +/* Modify rd pointer to alternate reg to avoid corrupting original reg */
> +static void emit_sextw_alt(u8 *rd, u8 ra, struct rv_jit_context *ctx)
> +{
> + emit_sextw(ra, *rd, ctx);
> + *rd = ra;
> +}
> +
> +static void emit_zextw_alt(u8 *rd, u8 ra, struct rv_jit_context *ctx)
> +{
> + emit_zextw(ra, *rd, ctx);
> + *rd = ra;
> +}
> +
> /* Emit fixed-length instructions for address */
> static int emit_addr(u8 rd, u64 addr, bool extra_pass, struct rv_jit_context *ctx)
> {
> @@ -395,38 +408,6 @@ static void init_regs(u8 *rd, u8 *rs, const struct bpf_insn *insn,
> *rs = bpf_to_rv_reg(insn->src_reg, ctx);
> }
>
> -static void emit_zext_32_rd_rs(u8 *rd, u8 *rs, struct rv_jit_context *ctx)
> -{
> - emit_mv(RV_REG_T2, *rd, ctx);
> - emit_zextw(RV_REG_T2, RV_REG_T2, ctx);
> - emit_mv(RV_REG_T1, *rs, ctx);
> - emit_zextw(RV_REG_T1, RV_REG_T1, ctx);
> - *rd = RV_REG_T2;
> - *rs = RV_REG_T1;
> -}
> -
> -static void emit_sext_32_rd_rs(u8 *rd, u8 *rs, struct rv_jit_context *ctx)
> -{
> - emit_sextw(RV_REG_T2, *rd, ctx);
> - emit_sextw(RV_REG_T1, *rs, ctx);
> - *rd = RV_REG_T2;
> - *rs = RV_REG_T1;
> -}
> -
> -static void emit_zext_32_rd_t1(u8 *rd, struct rv_jit_context *ctx)
> -{
> - emit_mv(RV_REG_T2, *rd, ctx);
> - emit_zextw(RV_REG_T2, RV_REG_T2, ctx);
> - emit_zextw(RV_REG_T1, RV_REG_T2, ctx);
> - *rd = RV_REG_T2;
> -}
> -
> -static void emit_sext_32_rd(u8 *rd, struct rv_jit_context *ctx)
> -{
> - emit_sextw(RV_REG_T2, *rd, ctx);
> - *rd = RV_REG_T2;
> -}
> -
> static int emit_jump_and_link(u8 rd, s64 rvoff, bool fixed_addr,
> struct rv_jit_context *ctx)
> {
> @@ -1372,22 +1353,22 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
> rvoff = rv_offset(i, off, ctx);
> if (!is64) {
> s = ctx->ninsns;
> - if (is_signed_bpf_cond(BPF_OP(code)))
> - emit_sext_32_rd_rs(&rd, &rs, ctx);
> - else
> - emit_zext_32_rd_rs(&rd, &rs, ctx);
> + if (is_signed_bpf_cond(BPF_OP(code))) {
> + emit_sextw_alt(&rs, RV_REG_T1, ctx);
> + emit_sextw_alt(&rd, RV_REG_T2, ctx);
> + } else {
> + emit_zextw_alt(&rs, RV_REG_T1, ctx);
> + emit_zextw_alt(&rd, RV_REG_T2, ctx);
> + }
> e = ctx->ninsns;
> -
Please avoid changes like this.
> /* Adjust for extra insns */
> rvoff -= ninsns_rvoff(e - s);
> }
> -
Dito.
> if (BPF_OP(code) == BPF_JSET) {
> /* Adjust for and */
> rvoff -= 4;
> emit_and(RV_REG_T1, rd, rs, ctx);
> - emit_branch(BPF_JNE, RV_REG_T1, RV_REG_ZERO, rvoff,
> - ctx);
> + emit_branch(BPF_JNE, RV_REG_T1, RV_REG_ZERO, rvoff, ctx);
> } else {
> emit_branch(BPF_OP(code), rd, rs, rvoff, ctx);
> }
> @@ -1416,21 +1397,20 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
> case BPF_JMP32 | BPF_JSLE | BPF_K:
> rvoff = rv_offset(i, off, ctx);
> s = ctx->ninsns;
> - if (imm) {
> + if (imm)
> emit_imm(RV_REG_T1, imm, ctx);
> - rs = RV_REG_T1;
> - } else {
> - /* If imm is 0, simply use zero register. */
> - rs = RV_REG_ZERO;
> - }
> + rs = imm ? RV_REG_T1 : RV_REG_ZERO;
> if (!is64) {
> - if (is_signed_bpf_cond(BPF_OP(code)))
> - emit_sext_32_rd(&rd, ctx);
> - else
> - emit_zext_32_rd_t1(&rd, ctx);
> + if (is_signed_bpf_cond(BPF_OP(code))) {
> + emit_sextw_alt(&rd, RV_REG_T2, ctx);
> + /* rs has been sign extended */
> + } else {
> + emit_zextw_alt(&rd, RV_REG_T2, ctx);
> + if (imm)
> + emit_zextw(rs, rs, ctx);
> + }
> }
> e = ctx->ninsns;
> -
Dito.
Other than the formatting changes, it looks good!
Björn
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