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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Fri, 9 Sep 2022 17:26:36 -0700
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Subject: Re: [PATCH] libbpf: define bpf_tail_call_static when __clang__ is not defined
To:     James Hilliard <james.hilliard1@gmail.com>
Cc:     bpf@vger.kernel.org, Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        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>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev
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On Fri, Sep 9, 2022 at 3:46 PM James Hilliard <james.hilliard1@gmail.com> wrote:
>
> The bpf_tail_call_static function is currently not defined unless
> using clang >= 8.
>
> To support bpf_tail_call_static on GCC we can check if __clang__ is
> not defined to enable bpf_tail_call_static.
>
> Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> ---
>  tools/lib/bpf/bpf_helpers.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> index 7349b16b8e2f..30fc95e7cd76 100644
> --- a/tools/lib/bpf/bpf_helpers.h
> +++ b/tools/lib/bpf/bpf_helpers.h
> @@ -131,7 +131,7 @@
>  /*
>   * Helper function to perform a tail call with a constant/immediate map slot.
>   */
> -#if __clang_major__ >= 8 && defined(__bpf__)
> +#if (!defined(__clang__) || __clang_major__ >= 8) && defined(__bpf__)
>  static __always_inline void
>  bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
>  {
> @@ -139,8 +139,8 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
>                 __bpf_unreachable();
>
>         /*
> -        * Provide a hard guarantee that LLVM won't optimize setting r2 (map
> -        * pointer) and r3 (constant map index) from _different paths_ ending
> +        * Provide a hard guarantee that the compiler won't optimize setting r2
> +        * (map pointer) and r3 (constant map index) from _different paths_ ending
>          * up at the _same_ call insn as otherwise we won't be able to use the
>          * jmpq/nopl retpoline-free patching by the x86-64 JIT in the kernel
>          * given they mismatch. See also d2e4c1e6c294 ("bpf: Constant map key
> @@ -148,8 +148,8 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
>          *
>          * Note on clobber list: we need to stay in-line with BPF calling
>          * convention, so even if we don't end up using r0, r4, r5, we need
> -        * to mark them as clobber so that LLVM doesn't end up using them
> -        * before / after the call.
> +        * to mark them as clobber so that the compiler doesn't end up using
> +        * them before / after the call.
>          */
>         asm volatile("r1 = %[ctx]\n\t"
>                      "r2 = %[map]\n\t"

will this compile as is on GCC-BPF? I'm trying to understand what's
the point. Once GCC supports this ASM syntax we can add similar check
to __clang_major__, instead of allowing it for all GCC versions?

We must have done __clang_major__ check for a reason, old Clangs
probably had some problems compiling this. Maybe Daniel remembers?

> --
> 2.34.1
>