Custom CSRs

The manual states:

The eXtension interface enables extension of CPU with:

• [...]
• Custom CSRs and related instructions.

However, I don't really see something related to CSRs. What is the intended direction? Is it intended, that it is possible to add CSRs in the custom ranges of CSRs (e.g. 0x800-0x8FF for M-mode R/W CSRs). As those would be addressable by Zicsr, I think this would require a dedicated interface to add such CSRs to the core.

Opinions?