Re: [PATCH v2 3/6] powerpc64/bpf: Tailcall handling with trampolines

Hari Bathini
2026-01-17T10:39:26Z

On 14/01/26 5:14 pm, adubey@linux.ibm.com wrote:
> From: Abhishek Dubey <adubey@linux.ibm.com>
>
> The trampoline mechanism sets up its own stack frame and
> an additional dummy frame. We need to have additional JIT
> instructions handling tailcall dereferencing in the
> trampoline's context.
>
> We don't add the two stack frames pointed above, rather
> add space for tail_call_info at bottom in trampoline frame
> for ppc64. This makes the trampoline's frame consistent with
> layout of all other frames wrt tail_call_info offset.
>
> Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
> ---
> arch/powerpc/net/bpf_jit_comp.c | 83 ++++++++++++++++++++++-----------
> 1 file changed, 56 insertions(+), 27 deletions(-)
>
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 069a8822c30d..e3088cf089d1 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -606,33 +606,58 @@ static int invoke_bpf_mod_ret(u32 *image, u32 *ro_image, struct codegen_context
> return 0;
> }
>
> -static void bpf_trampoline_setup_tail_call_cnt(u32 *image, struct codegen_context *ctx,
> - int func_frame_offset, int r4_off)
> -{
> - if (IS_ENABLED(CONFIG_PPC64)) {
> - /* See bpf_jit_stack_tailcallinfo_offset() */
> - int tailcallcnt_offset = 7 * 8;
> -
> - EMIT(PPC_RAW_LL(_R3, _R1, func_frame_offset - tailcallcnt_offset));
> - EMIT(PPC_RAW_STL(_R3, _R1, -tailcallcnt_offset));
> - } else {
> - /* See bpf_jit_stack_offsetof() and BPF_PPC_TC */
> - EMIT(PPC_RAW_LL(_R4, _R1, r4_off));
> - }
> -}
> +/*
> + * Refer the label 'Generated stack layout' in this file for actual stack
> + * layout during trampoline invocation.
> + *
> + * Refer __arch_prepare_bpf_trampoline() for stack component details.
> + *
> + * The tailcall count/reference is present in caller's stack frame. Its required
> + * to copy the content of tail_call_info before calling the actual function
> + * to which the trampoline is attached.
> + *
> + */
>
> -static void bpf_trampoline_restore_tail_call_cnt(u32 *image, struct codegen_context *ctx,
> - int func_frame_offset, int r4_off)
> +static void bpf_trampoline_setup_tail_call_info(u32 *image, struct codegen_context *ctx,
> + int func_frame_offset,
> + int bpf_dummy_frame_size, int r4_off)
> {
> if (IS_ENABLED(CONFIG_PPC64)) {
> /* See bpf_jit_stack_tailcallinfo_offset() */

> - int tailcallcnt_offset = 7 * 8;
> + int tailcallinfo_offset = BPF_PPC_TAILCALL;

This offset update should have been part of patch#1

> + /*
> + * func_frame_offset = ...(1)
> + * bpf_dummy_frame_size + trampoline_frame_size
> + */
> + EMIT(PPC_RAW_LD(_R4, _R1, func_frame_offset));
> + EMIT(PPC_RAW_LD(_R3, _R4, -tailcallinfo_offset));
> +
> + /*
> + * Setting the tail_call_info in trampoline's frame
> + * depending on if previous frame had value or reference.
> + */
> + EMIT(PPC_RAW_CMPLWI(_R3, MAX_TAIL_CALL_CNT));
> + PPC_COND_BRANCH(COND_GT, CTX_NIA(ctx) + 8);
> + EMIT(PPC_RAW_ADDI(_R3, _R4, bpf_jit_stack_tailcallinfo_offset(ctx)));
> + /*
> + * From ...(1) above:
> + * trampoline_frame_bottom = ...(2)
> + * func_frame_offset - bpf_dummy_frame_size
> + *
> + * Using ...(2) derived above:
> + * trampoline_tail_call_info_offset = ...(3)
> + * trampoline_frame_bottom - tailcallinfo_offset
> + *
> + * From ...(3):
> + * Use trampoline_tail_call_info_offset to write reference of main's
> + * tail_call_info in trampoline frame.
> + */
> + EMIT(PPC_RAW_STL(_R3, _R1, (func_frame_offset - bpf_dummy_frame_size)
> + - tailcallinfo_offset));
>
> - EMIT(PPC_RAW_LL(_R3, _R1, -tailcallcnt_offset));
> - EMIT(PPC_RAW_STL(_R3, _R1, func_frame_offset - tailcallcnt_offset));
> } else {
> /* See bpf_jit_stack_offsetof() and BPF_PPC_TC */
> - EMIT(PPC_RAW_STL(_R4, _R1, r4_off));
> + EMIT(PPC_RAW_LL(_R4, _R1, r4_off));
> }
> }
>
> @@ -720,6 +745,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
> * LR save area [ r0 save (64-bit) ] | header
> * [ r0 save (32-bit) ] |
> * dummy frame for unwind [ back chain 1 ] --
> + * [ tail_call_info ] non optional - 64-bit powerpc
> * [ padding ] align stack frame
> * r4_off [ r4 (tailcallcnt) ] optional - 32-bit powerpc
> * alt_lr_off [ real lr (ool stub)] optional - actual lr

> @@ -801,8 +827,14 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
> }
> }
>
> - /* Padding to align stack frame, if any */
> - bpf_frame_size = round_up(bpf_frame_size, SZL * 2);
> + if (!(bpf_frame_size % (2 * SZL))) {
> + /* Stack is 16-byte aligned */
> + /* Room for padding followed by 64-bit tail_call_info */
> + bpf_frame_size += SZL + BPF_PPC_TAILCALL;
> + } else {
> + /* Room for 64-bit tail_call_info */
> + bpf_frame_size += BPF_PPC_TAILCALL;
> + }
>
> /* Dummy frame size for proper unwind - includes 64-bytes red zone for 64-bit powerpc */
> bpf_dummy_frame_size = STACK_FRAME_MIN_SIZE + 64;

This change assumes the size is at least 8-byte aligned which is
true today but better skip that assumption by not touching the
padding part. The above hunk could simply be:

diff --git a/arch/powerpc/net/bpf_jit_comp.c
b/arch/powerpc/net/bpf_jit_comp.c
index 5e976730b2f5..266cc6f17dcc 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c
@@ -795,6 +795,10 @@ static int __arch_prepare_bpf_trampoline(struct
bpf_tramp_image *im, void *rw_im
}
}

+ /* Save tailcall count pointer at the same offset on the stack
where subprogs expect it */
+ if ((flags & BPF_TRAMP_F_CALL_ORIG) && (flags &
BPF_TRAMP_F_TAIL_CALL_CTX))
+ bpf_frame_size += SZL;
+
/* Padding to align stack frame, if any */
bpf_frame_size = round_up(bpf_frame_size, SZL * 2);

Patch#2 is not complete without this change. Please fold this patch
into patch#2 itself.

- Hari