reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/82babd3c0b49c6c5f4bab201528761763c65e1b2
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommu_pt
        git checkout 82babd3c0b49c6c5f4bab201528761763c65e1b2
        # 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