Re: On inlining more helpers in the JITs or the verifier

[Puranjay Mohan <puranjay@kernel.org>]

Puranjay Mohan <puranjay@kernel.org> writes:

> Hi Everyone,
>
> While working on inlining bpf_get_smp_processor_id() in the ARM64 and
> RISCV JITs, I realized that these archs allow such optimizations because
> they keep some information like the per-cpu offset or the pointer to the
> task_struct in special system registers.
>
> So, I went through the list of all BPF helpers and made a list of
> helpers that we can inline in these JITs to make their usage much more
> optimized:
>
> I. ARM64 and RISC-V specific optimzations if inlined:
>
>     A) Because pointer to tast_struct is available in a register:
>         1. bpf_get_current_pid_tgid()
>         2. bpf_get_current_task()

Tried inlining bpf_get_current_task() on ARM64:

                  Before                                                                  After
                 --------                                                               --------

bpf_prog_6e2672bcc4451a42_trigger_get_current_task:                      bpf_prog_6e2672bcc4451a42_trigger_get_current_task:
; task = (struct task_struct *)bpf_get_current_task();                   ; task = (struct task_struct *)bpf_get_current_task();
  34:   mov     x10, #0xffffffffffff9838                                   34:   mrs     x7, sp_el0
  38:   movk    x10, #0x8027, lsl #16
  3c:   movk    x10, #0x8000, lsl #32
  40:   blr     x10
  44:   add     x7, x0, #0x0



In the non-inlined version there is a branch [blr x10] to:

0xffff800080279838 bpf_get_current_task:
                     <+0>:     mrs     x0, sp_el0
                     <+4>:     ret


So, we only need a single instruction after inlining!!

I just don't know the best way to benchmark this. In theory it looks
highly optimized.

Thanks,
Puranjay