reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/ca213f193f2c2aa9b43a653bc6466e1216f6e767
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommufd_noiommu
        git checkout ca213f193f2c2aa9b43a653bc6466e1216f6e767

        # 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 drivers/iommu/iommufd/