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

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

On 22/10/24 01:49, Yonghong Song wrote:
> 
> On 10/21/24 6:39 AM, Leon Hwang wrote:
>> In the x86_64 JIT, when calling a function, tailcall info is
>> propagated if
>> the program is tail_call_reachable, regardless of whether the function
>> is a
>> subprog, helper, or kfunc. However, this propagation is unnecessary for
>> not-tail_call_reachable subprogs, helpers, or kfuncs.
>>
>> The verifier can determine if a subprog is tail_call_reachable.
>> Therefore,
>> it can be optimized to only propagate tailcall info when the callee is
>> subprog and the subprog is actually tail_call_reachable.
>>
>> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
>> ---
>>   arch/x86/net/bpf_jit_comp.c | 4 +++-
>>   kernel/bpf/verifier.c       | 6 ++++++
>>   2 files changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index 06b080b61aa57..6ad6886ecfc88 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
>> @@ -2124,10 +2124,12 @@ st:            if (is_imm8(insn->off))
>>                 /* call */
>>           case BPF_JMP | BPF_CALL: {
>> +            bool pseudo_call = src_reg == BPF_PSEUDO_CALL;
>> +            bool subprog_tail_call_reachable = dst_reg;
>>               u8 *ip = image + addrs[i - 1];
>>                 func = (u8 *) __bpf_call_base + imm32;
>> -            if (tail_call_reachable) {
>> +            if (pseudo_call && subprog_tail_call_reachable) {
> 
> Why we need subprog_tail_call_reachable? Does
>     tail_call_reachable && psueudo_call
> work the same way?
> 

'tail_call_reachable && pseudo_call' works too. However, it will
propagate tailcall info to subprog even if the subprog is not
tail_call_reachable.

subprog_tail_call_reachable indicates the subprog requires tailcall info
from its caller.
So, 'pseudo_call && subprog_tail_call_reachable' is better.

Thanks,
Leon