reproduce (this is a W=1 build):
        # https://github.com/jgunthorpe/linux/commit/7323ffb6a7a00fcc9c21bcb5ded7119da356110c
        git remote add jgunthorpe https://github.com/jgunthorpe/linux
        git fetch --no-tags jgunthorpe iommufd_dmabuf
        git checkout 7323ffb6a7a00fcc9c21bcb5ded7119da356110c

        # 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 drivers/vfio/pci/