RE: FW: BPF_PSEUDO_CALL question

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Alexei Starovoitov [mailto:alexei.starovoitov@gmail.com]
> Sent: Tuesday, April 16, 2019 2:01 AM
> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>; bpf <bpf@vger.kernel.org>
> Subject: Re: FW: BPF_PSEUDO_CALL question
> 
> On Mon, Apr 15, 2019 at 5:13 AM Ananyev, Konstantin
> <konstantin.ananyev@intel.com> wrote:
> >
> > Hi Alexei,
> > I posted the question below on llvm-dev mailing list, but there is no answer so far.
> > As you seems the author of BPF_PSEUDO_CALL in LLVM (and linux kernel) thought might
> > be you know the answer?
> > I looked through the patches, but didn't spot anything myself.
> > Thanks
> > Konstantin
> >
> > -----Original Message-----
> > From: llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org] On Behalf Of Ananyev, Konstantin via llvm-dev
> > Sent: Wednesday, April 10, 2019 7:37 PM
> > To: llvm-dev@lists.llvm.org
> > Subject: [llvm-dev] BPF_PSEUDO_CALL question
> >
> >
> > Hi everyone,
> > with clang 6.0 and onwards, for the following code:
> > $ cat t6.c
> >
> > #include <stdint.h>
> >
> > extern int ffx1(const void *p);
> >
> > uint64_t entry(const void *p)
> > {
> >         return ffx1(p);
> > }
> >
> > clang -O2  -target bpfel -c t6.c
> > generates for the call BPF_PSEUDO_CALL instruction:
> > entry:
> >        0:       85 10 00 00 ff ff ff ff         call -1
> >        1:       67 00 00 00 20 00 00 00         r0 <<= 32
> >        2:       c7 00 00 00 20 00 00 00         r0 s>>= 32
> >        3:       95 00 00 00 00 00 00 00         exit
> >
> > Is there any way to force clang to generate proper BPF_CALL instruction,
> > i.e: 85 00 00 00 ff ff ff ff (as it did in older versions)?
> 
> older version of llvm generated broken code.
> there are only two call flavors:
> - call imm -> calling particular kernel helper where imm==helper_id

Ok but in the example above, is it possible tell to the llvm that
ffx1() is actually a kernel helper with id==X?

> - call pseudo +-off -> call another bpf function
> there is no arbitrary call support yet.