On Wed, Nov 24, 2021 at 2:43 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> 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.
