llvm dependencies in ebpf backend can cause cmake problems.

[ChrisDodd]

Pulling the latest p4c, when I run cmake I get some errors:

CMake Warning at p4c/backends/ebpf/CMakeLists.txt:107 (MESSAGE):
  Kernel version 4.10.0 too small, expected 4.16.0.  Ignoring ebpf kernel
  tests...

CMake Error at /usr/share/llvm-3.8/cmake/LLVMConfig.cmake:178 (include):
  include could not find load file:

    /usr/share/llvm/cmake/LLVMExports.cmake
Call Stack (most recent call first):
  p4c/backends/ebpf/CMakeLists.txt:114 (find_package)

CMake Error at /usr/share/llvm-3.8/cmake/LLVMConfig.cmake:181 (include):
  include could not find load file:

    /usr/share/llvm/cmake/LLVM-Config.cmake
Call Stack (most recent call first):
  p4c/backends/ebpf/CMakeLists.txt:114 (find_package)

If I comment out the find_package(LLVM) line in that last cmake file, everything configures, but some stuff is disabled.

The README.md file just says that llvm 3.7.1 or later is required. Is the problem llvm 3.8?

[antoninbas]

I had the same issue, I had to run cmake with -DENABLE_EBPF to get rid of it.

[mihaibudiu]

This seems to be a bug in the llvm package for some ubuntu versions. The makefile has a link to a stackoverflow post describing a fix. It would be nice if the find-package would not crash and would just return false...

[mihaibudiu]

Here is the link describing the problem and the solution:
https://stackoverflow.com/questions/38171543/error-when-using-cmake-with-llvm
It is in a pending PR.
I am not sure what to do about it in the makefile.

[fruffy]

BCC seems to have a similar problem. This issue discusses several more workarounds:
iovisor/bcc#492