reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/b223fd6ff7a3c77e15e53a2068a3906acf8f178b
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommufd_dmabuf
        git checkout b223fd6ff7a3c77e15e53a2068a3906acf8f178b

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um olddefconfig
        make W=1 O=build_dir ARCH=um SHELL=/bin/bash drivers/vfio/pci/