Re: [PATCH v2] libbpf: add GCC support for bpf_tail_call_static

[Andrii Nakryiko]

On Fri, Sep 9, 2022 at 11:23 AM James Hilliard
<james.hilliard1@xxxxxxxxx> wrote:
>
> On Fri, Sep 9, 2022 at 12:05 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Mon, Aug 29, 2022 at 2:05 PM James Hilliard
> > <james.hilliard1@xxxxxxxxx> 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@xxxxxxxxx>
> > > ---
> > > 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
>

Sorry, I don't understand the question. I'm talking about the above
snippet with "r1 = %[ctx]" vs "mov %%r1,%[ctx]". Seems like the rest
stayed the same. So this would be a "BPF specific assembly format"
case, right? I don't know what else could be different with GCC-BPF
assembly.

> >
> > 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
> > >