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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Sat, 16 Dec 2023 14:47:33 -0800
Message-ID: <CAADnVQ+E2jdjV8=jy9ZktFT8Jvt=0_yKEoU6v9zqC52+prxPhQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next 2/3] bpf: selftests: Add bpf_assert_if() and
 bpf_assert_with_if() macros
To: Daniel Xu <dxu@dxuuu.xyz>
Cc: Alexei Starovoitov <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, 
	Andrii Nakryiko <andrii@kernel.org>, Shuah Khan <shuah@kernel.org>, 
	Kumar Kartikeya Dwivedi <memxor@gmail.com>, Martin KaFai Lau <martin.lau@linux.dev>, Song Liu <song@kernel.org>, 
	Yonghong Song <yonghong.song@linux.dev>, John Fastabend <john.fastabend@gmail.com>, 
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, 
	Jiri Olsa <jolsa@kernel.org>, Mykola Lysenko <mykolal@fb.com>, bpf <bpf@vger.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
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On Thu, Dec 14, 2023 at 7:10=E2=80=AFPM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> Just realized that we can go the other way instead.
>
> We can get rid of bpf_assert_eq/ne/... and replace with:
>
> diff --git a/tools/testing/selftests/bpf/bpf_experimental.h
> b/tools/testing/selftests/bpf/bpf_experimental.h
> index 1386baf9ae4a..1c500287766d 100644
> --- a/tools/testing/selftests/bpf/bpf_experimental.h
> +++ b/tools/testing/selftests/bpf/bpf_experimental.h
> @@ -254,6 +254,15 @@ extern void bpf_throw(u64 cookie) __ksym;
>                 }
>                  \
>          })
>
> +#define _EQ(LHS, RHS) \
> +       ({ int var =3D 1;\
> +               asm volatile goto("if %[lhs] =3D=3D %[rhs] goto %l[l_yes]=
" \
> +               :: [lhs] "r"(LHS), [rhs] "i"(RHS) :: l_yes);\
> +       var =3D 0;\
> +l_yes:\
> +       var;\
> +       })

Realized we can do much better.
We can take advantage that bpf assembly syntax resembles C and do:

bpf_assert(CMP(cookie, "!=3D", 0);

and use it as generic "volatile compare" that compiler cannot optimize out:

Replacing:
if (foo < bar) ...
with
if (CMP(foo, "<", bar)) ...
when the compare operator should be preserved.
I'll try to prototype it soon.

Might go further and use C++ for bpf programs :)
Override operator<, opreator=3D=3D, ...
then if (foo < bar) will be in asm code as written in C++ bpf prog.