openjdk-21-jre-zero 21~3ea-1 (riscv64 binary) in ubuntu lunar

 The package provides an alternative runtime using the Zero VM. Built on
 architectures in addition to the Hotspot VM as a debugging aid for those
 architectures which don't have a Hotspot VM.
 .
 The VM is started with the option `-zero'. See the README.Debian for details.