linux-kernel - Re: [PATCH v2] libbpf: add GCC support for bpf_tail_call

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Date:   Fri, 9 Sep 2022 12:22:47 -0600
From:   James Hilliard <james.hilliard1@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        "Jose E. Marchesi" <jose.marchesi@...cle.com>,
        "Jose E. Marchesi" <jemarch@....org>,
        David Faust <david.faust@...cle.com>
Cc:     bpf@...r.kernel.org, Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v2] libbpf: add GCC support for bpf_tail_call_static

On Fri, Sep 9, 2022 at 12:05 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Mon, Aug 29, 2022 at 2:05 PM James Hilliard
> <james.hilliard1@...il.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.
> >
> > We need to use GCC assembly syntax when the compiler does not define
> > __clang__ as LLVM inline assembly is not fully compatible with GCC.
> >
> > Signed-off-by: James Hilliard <james.hilliard1@...il.com>
> > ---
> > Changes v1 -> v2:
> >   - drop __BPF__ check as GCC now defines __bpf__
> > ---
> >  tools/lib/bpf/bpf_helpers.h | 19 +++++++++++++------
> >  1 file changed, 13 insertions(+), 6 deletions(-)
> >
> > diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> > index 7349b16b8e2f..867b734839dd 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
> > @@ -148,12 +148,19 @@ 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"
> > +       asm volatile(
> > +#ifdef __clang__
> > +                    "r1 = %[ctx]\n\t"
> >                      "r2 = %[map]\n\t"
> >                      "r3 = %[slot]\n\t"
> > +#else
> > +                    "mov %%r1,%[ctx]\n\t"
> > +                    "mov %%r2,%[map]\n\t"
> > +                    "mov %%r3,%[slot]\n\t"
> > +#endif
>
> Hey James,
>
> I don't think it's a good idea to have a completely different BPF asm
> syntax in GCC-BPF vs what Clang is supporting. Note that Clang syntax
> is also what BPF users see in BPF verifier log and in llvm-objdump
> output, so that's what BPF users are familiar with.

Is the difference a BPF specific assembly format deviation or a generic
deviation in assembler template syntax between GCC/llvm?
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#AssemblerTemplate

>
> This will cause constant and unavoidable maintenance burden both for
> libraries like libbpf and end users and their BPF apps as well.
>
> Given you are trying to make GCC-BPF part of the BPF ecosystem, please
> think about how to help the ecosystem, move it forward and unify it,
> not how to branch out and have Clang vs GCC differences everywhere.
> There is a lot of embedded BPF asm in production applications, having
> to write something as trivial as `r1 = X` in GCC or Clang-specific
> ways is a huge burden.
>
> As such, we've reverted your patch ([0]). Please add de facto BPF asm
> syntax support to GCC-BPF and this change won't be necessary.
>
>   [0] https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=665f5d3577ef43e929d59cf39683037887c351bf
>
> >                      "call 12"
> >                      :: [ctx]"r"(ctx), [map]"r"(map), [slot]"i"(slot)
> >                      : "r0", "r1", "r2", "r3", "r4", "r5");
> > --
> > 2.34.1
> >