reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/ade751a31a300ecb4bba4db3a4bc6e684f9a2e04
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommu_pt
        git checkout ade751a31a300ecb4bba4db3a4bc6e684f9a2e04
        # 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