reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/2edad14f2cea09830a4fbaf4352834f474ce4394
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommufd_dmabuf
        git checkout 2edad14f2cea09830a4fbaf4352834f474ce4394
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash