Re: [PATCH v2] PowerPC: Initial SFrame support for 64-bit PowerPC ELF ABI

On 2026-04-29 05:55, Abhay Kandpal wrote:
> Hi Indu,
> 
> Thanks for the review and for raising the question about motivation.
> 
> Regarding testing: yes, perf report based testing was done on both 
> ppc64le and ppc64 (BE). On BE, missing frames were observed under perf 
> profiling, consistent with what was seen on LE.
> 
> For the summary of why SFrame is necessary for PowerPC64, two root 
> problems were identified:
> 
> 1. Stale Link Register (LR) at NMI boundary
>     When a perf NMI fires, the kernel records the current LR value. If 
> the interrupted function has already called a sub-function, LR holds the 
> sub-function's return address — a stale value.
>     Currently, arch_skip_callchain_idx() in userspace perf 
> reads .eh_frame to detect and prune this stale entry,
>     but this is a fragile heuristic - SFrame in the kernel would allow 
> the unwinder to resolve this directly without relying on userspace post- 
> processing.
> 
> 2. Epilogue-race / missing caller
>     PPC64 epilogues restore SP with "addi r1,r1,<framesize>" before the 
> "blr" instruction.
>     If an NMI fires in this window, SP already points to the caller's 
> frame, so the backchain walk skips the interrupted function's direct 
> caller.
>     This was reproduced on both ppc64le and ppc64 BE. No userspace 
> heuristic can recover this lost frame — it requires kernel-side 
> unwinding with reliable frame data, which SFrame enables.
> 
> As a supplementary finding: we also tested the signal handler context. 
> The kernel's perf_callchain_user_64() handles signal frames correctly.
> However, arch_skip_callchain_idx() was observed incorrectly pruning a 
> valid LR entry at signal frame boundaries, causing the signal handler to 
> go missing from call chains.
> Although it can be fix seperately but this is another instance 
> of .eh_frame-based heuristics breaking down, they do not model signal 
> frame semantics and produce false positives at these boundaries.
> 
> Looking forward to your second-look review.
> 
> Thanks,
> Abhay
> 


Hi Abhay,

Since its been a while since you posted V2 (and I did not get time to 
get back to your patch), Is PowerPC/SFrame support still of interest ?

Thanks
Indu