User:Demindiro/SBI

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The RISC-V SBI (Supervisor Binary Interface) defines a common interface for RISC-V platforms for OSes. It hides platform-specific implementation details such that OSes are more portable.

Notably, it adds methods to facilitate IPI (Inter-Processor Interrupts).

The reference implementation is OpenSBI.

OpenSBI on QEMU

To run OpenSBI on QEMU, download the source and compile with make PLATFORM=generic CROSS_COMPILE=riscv64-your-os-. Then add -bios path/to/opensbi/build/platform/generic/firmware/fw_jump.bin

fw_jump vs fw_payload vs fw_dynamic

• fw_jump should be used in conjunction with QEMU's -kernel option

• fw_payload should directly include the kernel binary.

• fw_dynamic should be used if you use another bootloader that passes more information to OpenSBI.

Interface

The calling convention is the RISC-V ELF psABI. This means you'll need to save caller-saved registers!

The main difference is that the EID (Extension ID) goes into a7 and FID (Function ID) into a6.

Extensions

Avoid the legacy extensions (EID 0x00 - 0xFF) as those are deprecated.

Timer (0x54494D45)

The timer extension has a single function sbi_set_timer with FID 0x00. It takes a single uint64_t argument.

Before calling it, ensure that STIE is cleared! Otherwise you may get sporadic interrupts. i.e:

li	s0, 1 << 5
csrc	sie, s0
li	a7, 0x54494d45
li	a6, 0
ecall	
csrs	sie, s0

See Also

External Links

• RISC-V Supervisor Binary Interface Specification