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 at https://github.com/riscv/riscv-sbi-doc.

The goal of the OpenSBI project is to provide an open-source reference
implementation of the RISC-V SBI specifications for platform-specific firmwares
executing in M-mode (case 1 mentioned above). An OpenSBI implementation can be
easily extended by RISC-V platform and system-on-chip vendors to fit a
particular hardware configuration.