RISC-V Proxy Kernel

The RISC-V Proxy Kernel (pk) is a minimal, user-mode execution environment used to run RISC-V binaries on simulators and emulators. In the Swimmer-RISCV toolchain it serves two roles:

Application loader — When running an ELF binary compiled with riscv64-unknown-elf-gcc, the proxy kernel is passed to Swimmer-RISCV through the --use-pk option. The simulator loads the binary and the proxy kernel together so that the program can execute as if it were running natively under a Unix-like environment.
Berkeley Boot Loader (bbl) — When simulating RISC-V Linux built with Freedom-U-SDK, the proxy kernel is produced as freedom-u-sdk/work/riscv-pk/bbl and passed to Swimmer-RISCV via --binfile to boot the Linux image.

Usage with Swimmer-RISCV

The proxy kernel is integrated with Swimmer-RISCV through the following CLI options (taken from the Swimmer-RISCV README):

--use-pk <path> — Specify the location of the RISC-V Proxy Kernel.
--binfile <path> — RISC-V binary file to be simulated.
--hexfile <path> — RISC-V srec (hex) file to be simulated.
--stop-host — Simulator stops when accessing tohost/fromhost memory addresses (the proxy kernel's host-call interface).
--vmlinux <path> — Specify location of the vmlinux object, used for simulating RISC-V Linux (often built on top of riscv-pk/bbl).

Running a user program

$ cat test.c
#include <stdio.h>
main () { printf("Hello\n"); }

$ riscv64-unknown-elf-gcc test.c -o test.elf
$ swimmer_riscv --binfile test.elf --use-pk ${RISCV}/riscv64-unknown-elf/bin/pk
Hello

Booting Freedom-U-SDK Linux

$ swimmer_riscv --binfile freedom-u-sdk/work/riscv-pk/bbl

Relationship to Swimmer-RISCV

Swimmer-RISCV is a configurable RISC-V instruction set simulator written in C++. It depends on riscv-pk for:

Hosting user-mode RISC-V ELF programs (via --use-pk).
Providing the Berkeley Boot Loader that wraps a Linux kernel image for booting via --binfile (and optionally --vmlinux).

The proxy kernel therefore acts as the primary bridge between compiled RISC-V binaries and the Swimmer-RISCV simulator, enabling both application-level and full-Linux simulation flows.