Comment by jesse_m

For what it's worth, the current draft of the hypervisor extensions is available here: https://github.com/riscv/riscv-isa-manual/releases/download/... and is Chapter 5

It does say this: The hypervisor extension has been designed to be efficiently emulable on platforms that donot implement the extension, by running the hypervisor in S-mode and trapping into M-modefor hypervisor CSR accesses and to maintain shadow page tables. The majority of CSR accessesfor type-2 hypervisors are valid S-mode accesses so need not be trapped. Hypervisors can supportnested virtualization analogously.