The RISC-V Supervisor Binary Interface (SBI) is the recommended interface
between:

1. A platform-specific firmware running in M-mode and a bootloader, a
hypervisor or a general-purpose OS executing in S-mode or HS-mode.
2. A hypervisor running in HS-mode and a bootloader or a general-purpose
OS executing in VS-mode.

The RISC-V SBI specification is maintained as an independent project by
the RISC-V Foundation on Github (https://github.com/riscv/riscv-sbi-doc).