reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/2c39e478f1bbc1680ad47e8b418bbd76701a4496
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommu_pt
        git checkout 2c39e478f1bbc1680ad47e8b418bbd76701a4496
        # 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/