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

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Message-ID: <CABg9mcv8ekAxy98VZqq-Aev3rv3wx7UJDM++6AjOJqscJUN5NQ@mail.gmail.com>
Date:	Mon, 16 May 2016 16:45:19 -0700
From:	Z Lim <zlim.lnx@...il.com>
To:	Yang Shi <yang.shi@...aro.org>
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

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)

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

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(-)
>