Subject: Re: [PATCH net-next 2/3] arm64: bpf: optimize JMP_CALL

From: Will Deacon <[email-masked]>
Date: Tue, 7 Jun 2016 09:10:43 +0100

On Mon, Jun 06, 2016 at 09:36:03PM -0700, Z Lim wrote:
> On Mon, Jun 6, 2016 at 10:05 AM, Will Deacon <[email-masked]> wrote:
> > On Sat, Jun 04, 2016 at 03:00:29PM -0700, Zi Shen Lim wrote:
> >> Remove superfluous stack frame, saving us 3 instructions for
> >> every JMP_CALL.
> >>
> >> Signed-off-by: Zi Shen Lim <[email-masked]>
> >> ---
> >>  arch/arm64/net/bpf_jit_comp.c | 3 ---
> >>  1 file changed, 3 deletions(-)
> >>
> >> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> >> index 51abc97..7ae304e 100644
> >> --- a/arch/arm64/net/bpf_jit_comp.c
> >> +++ b/arch/arm64/net/bpf_jit_comp.c
> >> @@ -578,11 +578,8 @@ emit_cond_jmp:
> >>               const u64 func = (u64)__bpf_call_base + imm;
> >>
> >>               emit_a64_mov_i64(tmp, func, ctx);
> >> -             emit(A64_PUSH(A64_FP, A64_LR, A64_SP), ctx);
> >> -             emit(A64_MOV(1, A64_FP, A64_SP), ctx);
> >>               emit(A64_BLR(tmp), ctx);
> >>               emit(A64_MOV(1, r0, A64_R(0)), ctx);
> >> -             emit(A64_POP(A64_FP, A64_LR, A64_SP), ctx);
> >>               break;
> >>       }
> >
> > Is the jitted code intended to be unwindable by standard tools?
> 
> Before this patch:
>     bpf_prologue => push stack frame
>     ...
>     jmp_call => push stack frame, call bpf_helper*, pop stack frame
>     ...
>     bpf_epilogue => pop stack frame, ret
> 
> Now:
>     bpf_prologue => push stack frame
>     ...
>     jmp_call => call bpf_helper*
>     ...
>     bpf_epilogue => pop stack frame, ret
> 
> *Note: bpf_helpers in kernel/bpf/helper.c
> 
> So yes, it's still unwindable.

Sure, I'm not disputing that. I just wondered whether or not it needs to
be unwindable at all...

Will