reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/8aa0df6ce0d35430baeac4ee5e0ea77f82cf2ead
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe for-vasant
        git checkout 8aa0df6ce0d35430baeac4ee5e0ea77f82cf2ead
        # 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/