reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/david/linux.git/commit/?id=67723794142d332a8dc8ed6a8b5348b97703979f
        git remote add david https://git.kernel.org/pub/scm/linux/kernel/git/david/linux.git
        git fetch --no-tags david guestmemfd-preview
        git checkout 67723794142d332a8dc8ed6a8b5348b97703979f
        # apt-get install sparse
        # sparse version: v0.6.4-71-g610d841d

        # 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