reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/2a6bb518816bf74d80f8249e8c32b886f228df43
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommu_pt
        git checkout 2a6bb518816bf74d80f8249e8c32b886f228df43
        # 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