Re: [PATCH V3 2/2] arm64: bpf: make BPF prologue and epilogue align with ARM64 AAPCS

[Shi, Yang]

On 11/13/2015 6:39 PM, Z Lim wrote:
> Yang, I noticed another thing...
>
> On Fri, Nov 13, 2015 at 10:09 AM, Yang Shi <yang.shi@xxxxxxxxxx> wrote:
> > Save and restore FP/LR in BPF prog prologue and epilogue, save SP to FP
> > in prologue in order to get the correct stack backtrace.
> >
> > However, ARM64 JIT used FP (x29) as eBPF fp register, FP is subjected to
> > change during function call so it may cause the BPF prog stack base address
> > change too.
> >
> > Use x25 to replace FP as BPF stack base register (fp). Since x25 is callee
> > saved register, so it will keep intact during function call.
>
> Can you please add save/restore for x25 also? :)

Sure. BTW, since PUSH invokes stp instruction and SP need 16-bytes 
alignment, so we have to save x26 with x25 together. Anyway, it won't 
introduce any harm overhead since one instruction saves two registers.

Yang

> > It is initialized in BPF prog prologue when BPF prog is started to run
> > everytime. When BPF prog exits, it could be just tossed.

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