Re: [PATCH bpf-next 1/3] bpf, x64: Call perf_snapshot_branch_stack in trampoline

[Leon Hwang]

On 2026/1/10 00:24, Alexei Starovoitov wrote:
> On Fri, Jan 9, 2026 at 7:37 AM Leon Hwang <leon.hwang@xxxxxxxxx> wrote:
>>

[...]

>> @@ -3366,6 +3416,14 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
>>
>>         save_args(m, &prog, regs_off, false, flags);
>>
>> +       if (bpf_prog_copy_branch_snapshot(fentry)) {
>> +               /* Get branch snapshot asap. */
>> +               if (invoke_branch_snapshot(&prog, image, rw_image)) {
>> +                       ret = -EINVAL;
>> +                       goto cleanup;
>> +               }
>> +       }
> 
> Andrii already tried to do it.
> I hated it back then and still hate the idea.
> We're not going to add custom logic for one specific use case
> no matter how appealing it sounds to save very limited LBR entries.
> The HW will get better, but we will be stuck with this optimization forever.
> 

Understood, thanks for the explanation.

I won’t pursue this approach further.

Thanks,
Leon