reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/fad3fdebd11ef61f1991deea8761fde681b2fec3
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe smmuv3_nesting
        git checkout fad3fdebd11ef61f1991deea8761fde681b2fec3
        # 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 drivers/iommu/iommufd/