linux-kernel - Re: [PATCH net-next 2/3] arm64: bpf: optimize JMP

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Date:	Mon, 6 Jun 2016 21:36:03 -0700
From:	Z Lim <zlim.lnx@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Catalin Marinas <catalin.marinas@....com>,
	Yang Shi <yang.shi@...aro.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Network Development <netdev@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 2/3] arm64: bpf: optimize JMP_CALL

Hi Will,

On Mon, Jun 6, 2016 at 10:05 AM, Will Deacon <will.deacon@....com> 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 <zlim.lnx@...il.com>
>> ---
>>  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.

>
> Will