Re: [PATCH bpf-next v5 07/16] bpf: Reuse MAX_BPF_FUNC_ARGS for maximum number of arguments

[Yonghong Song <yonghong.song@linux.dev>]

On 4/16/26 9:30 PM, bot+bpf-ci@kernel.org wrote:
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -1151,6 +1151,10 @@ struct bpf_prog_offload {
>>
>>   /* The longest tracepoint has 12 args.
>>    * See include/trace/bpf_probe.h
>> + *
>> + * Also reuse this macro for maximum number of arguments a BPF function
>> + * or a kfunc can have. Args 1-5 are passed in registers, args 6-12 via
>> + * stack arg slots.
> Is this description of the calling convention accurate? The comment states
> "Args 1-5 are passed in registers, args 6-12 via stack arg slots," but
> doesn't argument 6 get passed in the R9 register rather than on the stack?
>
> Looking at arch/x86/net/bpf_jit_comp.c, the existing comment states: "x86-64
> supports up to MAX_BPF_FUNC_ARGS arguments. 1-6 are passed through regs, the
> remains are through stack."
>
> The follow-up commit 0ef907c3ec2c in this patch series ("bpf,x86: Implement
> JIT support for stack arguments") documents the actual calling convention as:
> "Follow x86-64 calling convention for both BPF-to-BPF and kfunc calls: - Arg
> 6 is passed in the R9 register - Args 7+ are passed on the stack at [rsp]"
>
> Should the comment say "Args 1-5 are passed in registers, arg 6 in R9
> register, args 7-12 via stack arg slots" instead?

I add something like:

The JIT may map some stack arg slots to registers based on the native calling
convention (e.g., arg 6 to R9 on x86-64).

>
>>    */
>>   #define MAX_BPF_FUNC_ARGS 12
>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/24546989054