reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/7ed632ebae4de991a280472e711b204da244ba74
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommu_pt_vtd
        git checkout 7ed632ebae4de991a280472e711b204da244ba74
        # 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/generic_pt/fmt/