Re: [PATCH bpf-next 07/12] bpf, x86: Place kfunc arguments per the SysV calling convention

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Thu Sep 3, 2026 at 10:10 PM PDT, Yonghong Song wrote:
> The JIT hands each eightbyte the BPF calling convention passes an
> argument in to the argument position of the same number, registers first,
> so the two conventions agree unless the kernel one places an argument
> somewhere else. Compute where SysV wants each eightbyte, and move the
> ones that differ before the call.
>
> SysV disagrees over an argument that the registers left cannot hold: it
> moves the whole of it to the stack and leaves the registers to the
> arguments that follow, while the BPF convention splits it and keeps
> filling slots in order. So for
>
>   u64 f(u64 a, u64 b, u64 c, u64 d, u64 e, struct pair s);
>
> the BPF convention puts s in the last argument register and the first
> stack slot, while SysV puts it wholly on the stack. Add an argument after
> s and it takes the register s vacated, which makes the moves a cycle, so
> one value at a time waits in RAX, dead before a call.
>
> The outgoing argument area is sized for both conventions, as SysV can put
> on the stack an argument the BPF slots kept in a register, and
> bpf_jit_supports_kfunc_arg_slot() can now answer yes to any placement.

What is SysV ? This change is about x86-64 ABI.
How come BPF calling convention diverged from x86-64?
Maybe we should adjust bpf side instead.
Looks like next patch is doing the same shift/move dance for arm64
which is a sign that we got it wrong on bpf side.
It needs to match x86/arm64.

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