netdev - status of llvm BTF support

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Date:   Mon, 17 Dec 2018 22:45:39 +0000
From:   Yonghong Song <yhs@...com>
To:     netdev <netdev@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        "Kernel Team" <Kernel-team@...com>
Subject: status of llvm BTF support

Hi,

As many of you already knew, latest bpf-next/net-next
kernel added supports for func_info and line_info.
   https://www.mail-archive.com/netdev@vger.kernel.org/msg255580.html
   https://www.mail-archive.com/netdev@vger.kernel.org/msg257914.html
   https://www.mail-archive.com/netdev@vger.kernel.org/msg272974.html

The func info and line info enables better ksym, correlation of
xlated/jited code with source code, and better source-annotated
verifier log. perf will be enhanced later to pick up
such debug info so better information can be displayed for hot
bpf programs.

Patch https://reviews.llvm.org/D53736
=====================================

Currently, the func info and line info are generated by LLVM
unmerged patch https://reviews.llvm.org/D53736. As stated in
the commit message of D53726, the main advantage of implementing
in LLVM is:
   . better integration/deployment as no extra tools are needed.
   . bpf JIT based compilation (like bcc, bpftrace, etc.) can get
     BTF without much extra effort.
   . BTF line_info needs selective source codes, which can be
     easily retrieved when inside the compiler.

Not in the above list is the future work related to
compile once and run everywhere (https://lwn.net/Articles/773198/),
which needs strong type. Such strong type is only available
in llvm IR. So implementation will have to be in llvm as well.

The llvm patch D53736 has been submitted on Oct 25, 2018.
The patch implemented BTF as a first class debug info, meaning
in a similar way to that Dwarf or CodeView debug info is generated.
The implementation is mainly placed under
   llvm/lib/CodeGen/AsmPrinter
directory. Since BTF currently is only used by BPF, cmake
file is changed to allow compilation of main BTFDebug.c only
if BPF is in the llvm build target list. Otherwise, a
stub implementation is used.

After initial review by Adrian Prantl where all comments
are addressed, the patch became dormand since Nov 16, except some
minor stylish review by Fangrui Song.

Patch https://reviews.llvm.org/{D55755,D55756,D55752}
=====================================================

During review of D53736, on Nov 16, I commented it was possible
to move main file BTFDebug.cpp to lib/Target/BPF
directory if moving some DebugInfo private headers
to llvm:include/llvm/CodeGen directory. But I never
got an answer whether this is a good idea or not.

Since review has dragged so long, to help facilitate discussion,
last weekend, I went ahead and implemented this scheme
with three patches in phabricator:
   https://reviews.llvm.org/D55755: refactor to move three DebugInfo
      private files into llvm:include/llvm/CodeGen directory.
   https://reviews.llvm.org/D55756: expose some private AsmPrinter
      members to protected so target extended AsmPrinter can use
      it to add their own DebugInfo handlers.
   https://reviews.llvm.org/D55752
      everything else is inside BPF directory.

This approach kept BTF completely under lib/Target/BPF
if patches D55755 and D55756 are implemented to enable
target specific debug info handlers.

Recap of two Approaches
=======================

To recap,
   https://reviews.llvm.org/D53736:
      everything in the llvm core, BTF is a "first-class" debug info.
      lib/Target/BPF unchanged.
   https://reviews.llvm.org/{D55755,D55756,D55752}:
      Some changes related DebugInfo to allow target specific
      debug info handlers.
      Major changes will be under /lib/Target/BPF.
At this point, it is not clear which approach will be accepted
by the community.

Going Forward
=============

We will continue to engage with LLVM community for integrating
BTF generation. Any help we can get from you should be helpful
and will be really appreciated.

Thanks,

Yonghong