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

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Date:	Mon, 16 May 2016 16:47:41 -0700
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Z Lim <zlim.lnx@...il.com>
Cc:	Alexei Starovoitov <ast@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Daniel Borkmann <daniel@...earbox.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH net-next] bpf: arm64: remove callee-save registers use for
 tmp registers

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@...aro.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@...il.com>
>> CC: Daniel Borkmann <daniel@...earbox.net>
>> Signed-off-by: Yang Shi <yang.shi@...aro.org>
>> ---
>
> Couple suggestions, but otherwise:
> Acked-by: Zi Shen Lim <zlim.lnx@...il.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(-)
>>