reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/dca06dac59465b43fe0a5fc6c9fa61107d3042c0
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommu_pt
        git checkout dca06dac59465b43fe0a5fc6c9fa61107d3042c0
        # 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