reproduce (this is a W=1 build):
        # https://github.com/sean-jc/linux/commit/3088b9660354afa55c95eeec0bba6d939123ce06
        git remote add sean-jc https://github.com/sean-jc/linux
        git fetch --no-tags sean-jc x86/vfio_kvm_file_refcount
        git checkout 3088b9660354afa55c95eeec0bba6d939123ce06

        # 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