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

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Date:   Sun, 28 Nov 2021 19:07:05 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.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 Thu, Nov 25, 2021 at 09:14:15AM -0700, Alexei Starovoitov wrote:
> 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.
> 

will change

thanks,
jirka