linux-kernel - Re: [PATCH 2/6] powerpc64/bpf: Tailcall handling with trampolines

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Message-ID: <7c2b6544-8a40-418a-81d5-b74494a36944@linux.ibm.com>
Date: Sat, 17 Jan 2026 16:03:27 +0530
From: Hari Bathini <hbathini@...ux.ibm.com>
To: adubey@...ux.ibm.com, bpf@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: sachinpb@...ux.ibm.com, venkat88@...ux.ibm.com, andrii@...nel.org,
        eddyz87@...il.com, mykolal@...com, ast@...nel.org,
        daniel@...earbox.net, martin.lau@...ux.dev, song@...nel.org,
        yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org,
        sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org,
        christophe.leroy@...roup.eu, naveen@...nel.org, maddy@...ux.ibm.com,
        mpe@...erman.id.au, npiggin@...il.com, memxor@...il.com,
        iii@...ux.ibm.com, shuah@...nel.org
Subject: Re: [PATCH 2/6] powerpc64/bpf: Tailcall handling with trampolines



On 05/01/26 4:22 pm, adubey@...ux.ibm.com wrote:
> From: Abhishek Dubey <adubey@...ux.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 conventional 'non-volatile register save area'
> and tail_call_info in trampoline's frame for ppc64. This
> makes the trampoline's frame consistent with layout of all
> other frames.
> 
> Signed-off-by: Abhishek Dubey <adubey@...ux.ibm.com>
> ---
>   arch/powerpc/net/bpf_jit_comp.c | 48 ++++++++++++++++++++++++++++-----
>   1 file changed, 42 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 069a8822c30d..4aaa0a287a45 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -606,15 +606,42 @@ 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)
> +/*
> + * 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_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;

This offset update should be part of patch#1

- Hari