reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/kbuild/linux.git/commit/?id=7b3281fcb43c5fce8d4d2b0996d3ac719cb5068b
        git remote add kbuild https://git.kernel.org/pub/scm/linux/kernel/git/kbuild/linux.git
        git fetch --no-tags kbuild kbuild-next-unstable
        git checkout 7b3281fcb43c5fce8d4d2b0996d3ac719cb5068b

        # 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/net/xen-netback/