reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/djiang/linux.git/commit/?id=23a9cfd0e8bf66157313427c66eb8cbdeafa0e31
        git remote add djiang https://git.kernel.org/pub/scm/linux/kernel/git/djiang/linux.git
        git fetch --no-tags djiang cxl/fwctl
        git checkout 23a9cfd0e8bf66157313427c66eb8cbdeafa0e31
        make W=1 O=/tmp/kselftest headers
        make W=1 O=/tmp/kselftest -C tools/testing/selftests
        make W=1 O=/tmp/kselftest -C tools/testing/selftests/bpf