reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/23dbb9b1e1ec4cc15b44eeb954b5045ed86daf78
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommu_pt
        git checkout 23dbb9b1e1ec4cc15b44eeb954b5045ed86daf78
        # 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/arm/arm-smmu-v3/