Hi, This is an update of llvm BTF support.
The patches corresponding to https://reviews.llvm.org/{D55755,D55756,D55752} have been accepted and merged. The llvm BTF support will be in llvm 8.0 whose release time is around next March. So with latest llvm/clang trunk, add -g to your compilation flag, .BTF and .BTF.ext will be generated. If you are curious about encoding, -S will produce an assembly code including .BTF/.BTF.ext. Thanks! Yonghong On 12/17/18 2:45 PM, Yonghong Song wrote: > 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 >
