copy the compiled libjffi-1.2.so to logstash-6.x.x/vendor/jruby/lib/jni/s390x-Linux/libjffi-1.2.so didn't worked for me(logstash issued a error message it cannot locate libjffi-1.2.so) what did the trick for me is:
cd logstash-6.x.x/logstash-core/lib/jars
copy the compiled libjffi-1.2.so to jni/s390x-Linux/libjffi-1.2.so
zip -g jruby-complete-9.1.13.0.jar jni/s390x-Linux/libjffi-1.2.so
copy the compiled libjffi-1.2.so to logstash-6.x.x/vendor/jruby/lib/jni/s390x-Linux/libjffi-1.2.so didn't worked for me(logstash issued a error message it cannot locate libjffi-1.2.so) what did the trick for me is: cd logstash-6.x.x/logstash-core/lib/jars copy the compiled libjffi-1.2.so to jni/s390x-Linux/libjffi-1.2.so zip -g jruby-complete-9.1.13.0.jar jni/s390x-Linux/libjffi-1.2.so