Re: [PATCH v2 1/6] powerpc64/bpf: Move tail_call_cnt to bottom of stack frame

Christophe Leroy (CS GROUP)
2026-01-15T08:29:20Z

Le 14/01/2026 à 12:44, adubey@linux.ibm.com a écrit :
> From: Abhishek Dubey <adubey@linux.ibm.com>
>
> In the conventional stack frame, the position of tail_call_cnt
> is after the NVR save area (BPF_PPC_STACK_SAVE). Whereas, the
> offset of tail_call_cnt in the trampoline frame is after the
> stack alignment padding. BPF JIT logic could become complex
> when dealing with frame-sensitive offset calculation of
> tail_call_cnt. Having the same offset in both frames is the
> desired objective.
>
> The trampoline frame does not have a BPF_PPC_STACK_SAVE area.
> Introducing it leads to under-utilization of extra memory meant
> only for the offset alignment of tail_call_cnt.
> Another challenge is the variable alignment padding sitting at
> the bottom of the trampoline frame, which requires additional
> handling to compute tail_call_cnt offset.
>
> This patch addresses the above issues by moving tail_call_cnt
> to the bottom of the stack frame at offset 0 for both types
> of frames. This saves additional bytes required by BPF_PPC_STACK_SAVE
> in trampoline frame, and a common offset computation for
> tail_call_cnt serves both frames.
>
> The changes in this patch are required by the third patch in the
> series, where the 'reference to tail_call_info' of the main frame
> is copied into the trampoline frame from the previous frame.
>
> Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
> ---
> arch/powerpc/net/bpf_jit.h | 4 ++++
> arch/powerpc/net/bpf_jit_comp64.c | 31 ++++++++++++++++++++-----------
> 2 files changed, 24 insertions(+), 11 deletions(-)
>
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index 8334cd667bba..45d419c0ee73 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -72,6 +72,10 @@
> } } while (0)
>
> #ifdef CONFIG_PPC64
> +
> +/* for tailcall counter */
> +#define BPF_PPC_TAILCALL 8

This needs to be defined outside of CONFIG_PPC64 ifdef because from
patch 3 it is used in bpf_jit_comp.c which is also built on powerpc32.

> +
> /* If dummy pass (!image), account for maximum possible instructions */
> #define PPC_LI64(d, i) do { \
> if (!image) \