Re: [PATCH v2] libbpf: define bpf_tail_call_static when __clang__ is not defined

[James Hilliard]

On Mon, Sep 12, 2022 at 11:29 AM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 9/12/22 7:35 AM, James Hilliard wrote:
> > The bpf_tail_call_static function is currently not defined unless
> > using clang >= 8.
> >
> > To support bpf_tail_call_static on GAS we can check if __clang__ is
> > not defined to enable bpf_tail_call_static.
> >
> > We need to use a GAS assembly syntax check so that the assembler
> > is provided GAS compatible assembly as well.
> >
> > We can use gasversion to provide a migration path to llvm syntax
> > for GAS once llvm syntax is natively supported.
>
> I didn't see a gasversion comparison in asm code.
> Is it possible that we compare gasversion to a known
> gas version which supports new syntax? If the gasversion
> is supported, use the same syntax as llvm. If the
> gasversion is not supported, output an illegal insn
> and it would be even better if some error information
> is printed out on the screen.

Yeah, once llvm syntax is supported in GAS the check would simply
need to be changed to something like:
.if .gasversion. < 24000

At least this seems to me to be the best way to provide a migration
path as we can't really check assembler versions from the compiler
like we can with llvm.

>
> >
> > Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
> > Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > ---
> > Changes v1 -> v2:
> >    - use gasversion to detect assembly variant
> > ---
> >   tools/lib/bpf/bpf_helpers.h | 18 ++++++++++++------
> >   1 file changed, 12 insertions(+), 6 deletions(-)
> >
> > diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> > index 7349b16b8e2f..5b98f5506798 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 +148,18 @@ 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 .gasversion.\n\t"
> > +                  "mov %%r1,%[ctx]\n\t"
> > +                  "mov %%r2,%[map]\n\t"
> > +                  "mov %%r3,%[slot]\n\t"
> > +                  ".else\n\t"
> > +                  "r1 = %[ctx]\n\t"
> >                    "r2 = %[map]\n\t"
> >                    "r3 = %[slot]\n\t"
> > +                  ".endif\n\t"
> >                    "call 12"
> >                    :: [ctx]"r"(ctx), [map]"r"(map), [slot]"i"(slot)
> >                    : "r0", "r1", "r2", "r3", "r4", "r5");