netdev - Re: [PATCH bpf-next 06/29] bpf: Add bpf_arg/bpf_ret

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Date:   Thu, 25 Nov 2021 09:14:15 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next 06/29] bpf: Add bpf_arg/bpf_ret_value helpers for
 tracing programs

On Wed, Nov 24, 2021 at 2:43 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
> > +               /* Implement bpf_arg inline. */
> > +               if (prog_type == BPF_PROG_TYPE_TRACING &&
> > +                   insn->imm == BPF_FUNC_arg) {
> > +                       /* Load nr_args from ctx - 8 */
> > +                       insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> > +                       insn_buf[1] = BPF_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 4);
> > +                       insn_buf[2] = BPF_ALU64_IMM(BPF_MUL, BPF_REG_2, 8);
> > +                       insn_buf[3] = BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1);
> > +                       insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_2, 0);
> > +                       insn_buf[5] = BPF_JMP_A(1);
> > +                       insn_buf[6] = BPF_MOV64_IMM(BPF_REG_0, 0);
> > +
> > +                       new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 7);
> > +                       if (!new_prog)
> > +                               return -ENOMEM;
> > +
> > +                       delta    += 6;
> > +                       env->prog = prog = new_prog;
> > +                       insn      = new_prog->insnsi + i + delta;
> > +                       continue;
>
> nit: this whole sequence of steps and calculations seems like
> something that might be abstracted and hidden behind a macro or helper
> func? Not related to your change, though. But wouldn't it be easier to
> understand if it was just written as:
>
> PATCH_INSNS(
>     BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
>     BPF_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 4);
>     BPF_ALU64_IMM(BPF_MUL, BPF_REG_2, 8);
>     BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1);
>     BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_2, 0);
>     BPF_JMP_A(1);
>     BPF_MOV64_IMM(BPF_REG_0, 0));

Daniel and myself tried to do similar macro magic in the past,
but it suffers unnecessary stack increase and extra copies.
So eventually we got rid of it.
I suggest staying with Jiri's approach.

Independent from anything else...
Just noticed BPF_MUL in the above...
Please use BPF_LSH instead. JITs don't optimize such things.
It's a job of gcc/llvm to do so. JITs assume that all
normal optimizations were done by the compiler.