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
>
> 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
> >
