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

Hari Bathini
2026-01-17T10:41:53Z

On 17/01/26 4:09 pm, Hari Bathini wrote:
>
>
> 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

Also, why non-optional? This can be optional on BPF_TRAMP_F_CALL_ORIG
and BPF_TRAMP_F_TAIL_CALL_CTX flags?

>>        *                              [ 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