reproduce:
        # https://github.com/intel/gvt-linux/commit/610b2bb74d663c7cc375650ffe6f8b285e18bff4
        git remote add intel-gvt https://github.com/intel/gvt-linux.git
        git fetch --no-tags intel-gvt pr/236
        git checkout 610b2bb74d663c7cc375650ffe6f8b285e18bff4
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 1.4.9
        make htmldocs