s390 aka IBM Z aka the mainframe has 80 years of pedigree. Since 20+ years it is supported in the Linux kernel. In many ways it is just another processor architecture. In some ways it has its specialities, especially when it comes to I/O and networking.

Let us tell you about multiple levels of Hipervisors and IO-virtualization. About sharing resources with non-Linux systems. About network cards that are not attached via PCI. About network interfaces that are implemented in Hardware, Firmware or Hipervisor software and protocols that serve special purposes or predate ethernet.