reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/8e43ac717967eb0f573622be728fc994e3942095
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommu_pt
        git checkout 8e43ac717967eb0f573622be728fc994e3942095
        # 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/