reproduce (this is a W=1 build):
        # https://github.com/agraf/linux-2.6/commit/45a0bb294baa814a4b7f4acbc0eea8c55b05cd69
        git remote add agraf-2.6 https://github.com/agraf/linux-2.6.git
        git fetch --no-tags agraf-2.6 virtio-dmb
        git checkout 45a0bb294baa814a4b7f4acbc0eea8c55b05cd69
        # apt-get install sparse
        # sparse version: v0.6.4-73-gfbdde312

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=x86_64 olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=x86_64 SHELL=/bin/bash drivers/virtio/