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

[Jose E. Marchesi]

> On Fri, Sep 9, 2022 at 12:56 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
>>
>> 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.
>
> I mean it's unclear if it's a generic assembly template format difference
> that applies to all targets or one that's BPF target specific.

It is certainly BPF specific.

I think that when I first wrote the BPF GNU toolchain port the assembly
format used by LLVM was different than it is now: I certainly didn't
invent the syntax the GNU assembler uses for BPF.

It seems LLVM settled with that C-like syntax for assembly instead,
which is very unconventional.

> Anyways for now I sent a new patch so that bpf_tail_call_static is defined
> on non-clang compilers so that it will work when GCC-BPF supports the
> existing asm format.
> https://lore.kernel.org/bpf/20220909224544.3702931-1-james.hilliard1@xxxxxxxxx/

I don't think this patch makes much sense: these guards are designed to
avoid compilers that do not support the inline assembly (as presumably
happen with clang < 8) to choke on the header file.  That's also the
case of GCC BPF at the moment.

With this patch, people won't be currentty able to use bpf_helpers.h
with GCC BPF even if they don't use bpf_tail_call_static.

>> > >
>> > > 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
>> > > >