netdev - Re: [PATCH bpf-next v2 3/9] riscv, bpf: add support for far branching when emitting tail call

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Date:   Mon, 23 Dec 2019 10:18:31 -0800 (PST)
From:   Palmer Dabbelt <palmerdabbelt@...gle.com>
To:     Bjorn Topel <bjorn.topel@...il.com>
CC:     daniel@...earbox.net, ast@...nel.org, netdev@...r.kernel.org,
        Bjorn Topel <bjorn.topel@...il.com>,
        linux-riscv@...ts.infradead.org, bpf@...r.kernel.org
Subject:     Re: [PATCH bpf-next v2 3/9] riscv, bpf: add support for far branching when emitting tail call

On Mon, 16 Dec 2019 01:13:37 PST (-0800), Bjorn Topel wrote:
> Start use the emit_branch() function in the tail call emitter in order
> to support far branching.
>
> Signed-off-by: Björn Töpel <bjorn.topel@...il.com>
> ---
>  arch/riscv/net/bpf_jit_comp.c | 22 +++-------------------
>  1 file changed, 3 insertions(+), 19 deletions(-)
>
> diff --git a/arch/riscv/net/bpf_jit_comp.c b/arch/riscv/net/bpf_jit_comp.c
> index e599458a9bcd..c38c95df3440 100644
> --- a/arch/riscv/net/bpf_jit_comp.c
> +++ b/arch/riscv/net/bpf_jit_comp.c
> @@ -496,16 +496,6 @@ static int is_12b_check(int off, int insn)
>  	return 0;
>  }
>
> -static int is_13b_check(int off, int insn)
> -{
> -	if (!is_13b_int(off)) {
> -		pr_err("bpf-jit: insn=%d 13b < offset=%d not supported yet!\n",
> -		       insn, (int)off);
> -		return -1;
> -	}
> -	return 0;
> -}
> -
>  static int is_21b_check(int off, int insn)
>  {
>  	if (!is_21b_int(off)) {
> @@ -744,18 +734,14 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
>  		return -1;
>  	emit(rv_lwu(RV_REG_T1, off, RV_REG_A1), ctx);
>  	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
> -	if (is_13b_check(off, insn))
> -		return -1;
> -	emit(rv_bgeu(RV_REG_A2, RV_REG_T1, off >> 1), ctx);
> +	emit_branch(BPF_JGE, RV_REG_A2, RV_REG_T1, off, ctx);
>
>  	/* if (--TCC < 0)
>  	 *     goto out;
>  	 */
>  	emit(rv_addi(RV_REG_T1, tcc, -1), ctx);
>  	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
> -	if (is_13b_check(off, insn))
> -		return -1;
> -	emit(rv_blt(RV_REG_T1, RV_REG_ZERO, off >> 1), ctx);
> +	emit_branch(BPF_JSLT, RV_REG_T1, RV_REG_ZERO, off, ctx);
>
>  	/* prog = array->ptrs[index];
>  	 * if (!prog)
> @@ -768,9 +754,7 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
>  		return -1;
>  	emit(rv_ld(RV_REG_T2, off, RV_REG_T2), ctx);
>  	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
> -	if (is_13b_check(off, insn))
> -		return -1;
> -	emit(rv_beq(RV_REG_T2, RV_REG_ZERO, off >> 1), ctx);
> +	emit_branch(BPF_JEQ, RV_REG_T2, RV_REG_ZERO, off, ctx);
>
>  	/* goto *(prog->bpf_func + 4); */
>  	off = offsetof(struct bpf_prog, bpf_func);

Reviewed-by: Palmer Dabbelt <palmerdabbelt@...gle.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@...gle.com>