reproduce (this is a W=1 build):
        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # https://git.kernel.org/pub/scm/linux/kernel/git/sandeen/linux.git/commit/?id=4a72a2292eafad1371bc73defec4fdea64ea6de7
        git remote add sandeen https://git.kernel.org/pub/scm/linux/kernel/git/sandeen/linux.git
        git fetch --no-tags sandeen convert-9p
        git checkout 4a72a2292eafad1371bc73defec4fdea64ea6de7
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-15.1.0 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=loongarch olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-15.1.0 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=loongarch SHELL=/bin/bash fs/9p/ net/9p/