Re: [PATCH net-next] bpf: arm64: remove callee-save registers use for tmp registers

From	"Shi, Yang" <yang.shi@linaro.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH net-next] bpf: arm64: remove callee-save registers use for tmp registers
Date	2016-05-17 01:50 +0200
Message-ID	<rzAqS-5dL-7@gated-at.bofh.it> (permalink)
References	<rzAhc-5aA-9@gated-at.bofh.it> <rzAqS-5dL-5@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On 5/16/2016 4:45 PM, Z Lim wrote:
> Hi Yang,
>
> On Mon, May 16, 2016 at 4:09 PM, Yang Shi <yang.shi@linaro.org> wrote:
>> In the current implementation of ARM64 eBPF JIT, R23 and R24 are used for
>> tmp registers, which are callee-saved registers. This leads to variable size
>> of JIT prologue and epilogue. The latest blinding constant change prefers to
>> constant size of prologue and epilogue. AAPCS reserves R9 ~ R15 for temp
>> registers which not need to be saved/restored during function call. So, replace
>> R23 and R24 to R10 and R11, and remove tmp_used flag.
>>
>> CC: Zi Shen Lim <zlim.lnx@gmail.com>
>> CC: Daniel Borkmann <daniel@iogearbox.net>
>> Signed-off-by: Yang Shi <yang.shi@linaro.org>
>> ---
>
> Couple suggestions, but otherwise:
> Acked-by: Zi Shen Lim <zlim.lnx@gmail.com>
>
> 1. Update the diagram. I think it should now be:
>
> -        * BPF fp register => -80:+-----+ <= (BPF_FP)
> +        * BPF fp register => -64:+-----+ <= (BPF_FP)

Nice catch. I forgot the stack diagram.

>
> 2. Add a comment in commit log along the lines of: this is an
> optimization saving 2 instructions per jited BPF program.

Sure, will address in V2.

Thanks,
Yang

>
> Thanks :)
>
> z
>
>> Apply on top of Daniel's blinding constant patchset.
>>
>>  arch/arm64/net/bpf_jit_comp.c | 32 ++++----------------------------
>>  1 file changed, 4 insertions(+), 28 deletions(-)
>>

Thread

[PATCH net-next] bpf: arm64: remove callee-save registers use for tmp registers Yang Shi <yang.shi@linaro.org> - 2016-05-17 01:40 +0200
  Re: [PATCH net-next] bpf: arm64: remove callee-save registers use for  tmp registers "Shi, Yang" <yang.shi@linaro.org> - 2016-05-17 01:50 +0200
  Re: [PATCH net-next] bpf: arm64: remove callee-save registers use for  tmp registers Z Lim <zlim.lnx@gmail.com> - 2016-05-17 01:50 +0200

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