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

[Shi, Yang]

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@xxxxxxxxxx> 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@xxxxxxxxx>
> > CC: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxx>
> > ---
>
> Couple suggestions, but otherwise:
> Acked-by: Zi Shen Lim <zlim.lnx@xxxxxxxxx>
>
> 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(-)