Comment 1 for bug 1998032

Thanks for reporting the issue.
The improper L3 cache size reporting seems more linked to kernel/lscpu bug/version and not libvirt/nova.
Quickly looking at Ubuntu requirements for Epyc: https://ubuntu.com/blog/amd-epyc-rome-support-in-ubuntu-server, it seems a minimum kernel 4.18.0-1017 version is required.
So could you please ensure you have at least this kernel version ? Although I'll encourage you to upgrade to the latest available kernel for your distribution ?

Also, have a look at this thread, as the issue looks closer to what your are experiencing: https://github.com/util-linux/util-linux/issues/663

Feel free to comment and change this bug to new again if you disagree or want to provide more info.