Re: [PATCH v2 bpf-next 16/20] bpf: Add helper macro bpf_arena_cast()

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

On Sat, Feb 10, 2024 at 12:54 AM Kumar Kartikeya Dwivedi
<memxor@gmail.com> wrote:
>
> On Fri, 9 Feb 2024 at 05:07, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > From: Alexei Starovoitov <ast@kernel.org>
> >
> > Introduce helper macro bpf_arena_cast() that emits:
> > rX = rX
> > instruction with off = BPF_ARENA_CAST_KERN or off = BPF_ARENA_CAST_USER
> > and encodes address_space into imm32.
> >
> > It's useful with older LLVM that doesn't emit this insn automatically.
> >
> > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > ---
>
> Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
>
> But could this simply be added to libbpf along with bpf_cast_user and
> bpf_cast_kern? I believe since LLVM and the verifier support the new
> cast instructions, they are unlikely to disappear any time soon. It
> would probably also make it easier to use elsewhere (e.g. sched-ext)
> without having to copy them.

This arena bpf_arena_cast() macro probably will be removed
once llvm 19 is released and we upgrade bpf CI to it.
It's here for selftests only.
It's quite tricky and fragile to use in practice.
Notice it does:
"r"(__var)
which is not quite correct,
since llvm won't recognize it as output that changes __var and
will use a copy of __var in a different register later.
But if the macro changes to "=r" or "+r" then llvm allocates
a register and that screws up codegen even more.

The __var;}) also doesn't always work.
So this macro is not suited for all to use.

> I plan on doing the same eventually with assert macros too.

I think it's too early to move them.