Re: [PATCH bpf-next v3 1/2] bpf, x64: Propagate tailcall info only for subprogs

[Leon Hwang <leon.hwang@linux.dev>]

On 13/11/24 09:31, Alexei Starovoitov wrote:
> On Thu, Nov 7, 2024 at 5:46 AM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>> In x64 JIT, propagate tailcall info only for subprogs, not for helpers
>> or kfuncs.
>>
>> Acked-by: Yonghong Song <yonghong.song@linux.dev>
>> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
>> ---
>>  arch/x86/net/bpf_jit_comp.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index 06b080b61aa57..eb08cc6d66401 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
>> @@ -2124,10 +2124,11 @@ st:                     if (is_imm8(insn->off))
>>
>>                         /* call */
>>                 case BPF_JMP | BPF_CALL: {
>> +                       bool pseudo_call = src_reg == BPF_PSEUDO_CALL;
>>                         u8 *ip = image + addrs[i - 1];
>>
>>                         func = (u8 *) __bpf_call_base + imm32;
>> -                       if (tail_call_reachable) {
>> +                       if (pseudo_call && tail_call_reachable) {
>>                                 LOAD_TAIL_CALL_CNT_PTR(bpf_prog->aux->stack_depth);
>>                                 ip += 7;
>>                         }
> 
> I've applied this patch with this tweak:
> if (src_reg == BPF_PSEUDO_CALL && tail_call_reachable)
> 
> I don't see much value in patch 2.
> The tail_call feature is an old approach. It is now causing
> maintenance issues with other features.
> I'd rather not touch anything tail call related.
> So I dropped patch 2.
> 
> I'd like to see proper indirect goto and indirect call
> support being developed further.
> Anton started working on it, but dropped the ball.
> We need to commandeer the patches.

Great to see jmp table supporting tail call.

Thanks,
Leon