Re: [PATCH v2 0/1] Add BPF callx support to objdump and as

[Will Hawkins <hawkinsw@obs.cr>]

Hello!

First, thank you for the response!

On Mon, Feb 12, 2024 at 1:39 PM Jose E. Marchesi
<jose.marchesi@oracle.com> wrote:
>
>
> Hi Will.
>
> [Adding Yonghong and Eduard in CC]
>
> > After additional consideration and discussion with Jose and Dave,
> > it seems like we have determined the way that clang, gcc and binutils
> > need to handle the callx/callr:
> >
> > 1. callr remains with the register holding the target of the jump stored
> > in the dst_reg.
> > 2. callx is added with the register holding the target of the jump stored
> > in the imm32.
> > 3. We have to remove the pseudoc syntax because it is no longer possible
> > to disambiguate between versions of call by simply looking at the
> > parameter.
>
> I don't recall reaching any agreement on the above.  What is the point
> of having both callr and callx?

Sorry! I was being slightly loose in terms of agreement -- I was
reading into your comments in the email between you, me and Dave from
earlier this weekend!

The only point in having both callr and callx was to allow the gcc
encoding to continue to exist in its current form. I assumed that
there was a compelling reason and certainly did not want to do
anything to interfere with the great work that you are doing!

>
> The existing callr is generated by GCC in -mxbpf mode.  It is an
> experimental extension that we use in order to be able to run more of
> the GCC testsuite, so it is always possible to change it to use imm32
> instead of dst_reg.
>
> I wouldn't personally welcome that change and would much prefer if clang
> starts using either reg_src or reg_dst, because compromising/reserving
> endian-dependent 32 whole bits for a register number that only requires
> 4 bits seems like a waste of insn space that will complicate future ISA
> extensions.

I 100% agree that it is less than ideal. However, it seems like the
cat is out of the bag. I am adding Dave who is leading the ISA
standardization effort. He and I (and others) have discussed this as
recently as this morning. I will let him weigh in on whether or not we
have the "power" to push back on clang's choice of how to encode the
instructions.

>
> In either case, if we all use the same encoding for the indirect call
> instruction (I fail to see any reason for not doing so) then point
> 3. becomes moot.

I agree and I really would like that to be the outcome. However, see
above (insert smiley face here!)

Thank you for responding!

Will

>
> >
> > Tests are added/refactored to meet the above.
> >
> > I am more than happy to resend as a separate mailing to the list but
> > sending first as a reply in order to keep list traffic manageable.
> >
> > As I said before, I sincerely appreciate all that you are doing for
> > the community and how welcoming you have been to a first-time contributor.
> >
> > Sincerely,
> > Will
> >
> > Will Hawkins (1):
> >   objdump, as: Add callx support for BPF CPU v1
> >
> >  gas/config/tc-bpf.c                           | 25 ++++++++++++++++++-
> >  gas/testsuite/gas/bpf/bpf.exp                 |  4 +--
> >  .../gas/bpf/{indcall-1.d => callr.d}          |  4 +--
> >  .../gas/bpf/{indcall-1.s => callr.s}          |  2 +-
> >  gas/testsuite/gas/bpf/indcall-1-pseudoc.d     | 23 -----------------
> >  gas/testsuite/gas/bpf/indcall-1-pseudoc.s     | 13 ----------
> >  include/opcode/bpf.h                          |  3 ++-
> >  opcodes/bpf-dis.c                             |  6 +++++
> >  opcodes/bpf-opc.c                             |  4 ++-
> >  sim/bpf/bpf-sim.c                             |  4 +++
> >  10 files changed, 44 insertions(+), 44 deletions(-)
> >  rename gas/testsuite/gas/bpf/{indcall-1.d => callr.d} (90%)
> >  rename gas/testsuite/gas/bpf/{indcall-1.s => callr.s} (90%)
> >  delete mode 100644 gas/testsuite/gas/bpf/indcall-1-pseudoc.d
> >  delete mode 100644 gas/testsuite/gas/bpf/indcall-1-pseudoc.s