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

[Hari Bathini <hbathini@linux.ibm.com>]

On 05/01/26 4:22 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 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@linux.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