reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/5849b503c1771ec07a34aca7fa51ef3788514f29
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommufd_dmabuf
        git checkout 5849b503c1771ec07a34aca7fa51ef3788514f29

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash