Multiplix kernel

Multiplix is a small operating system serving as the foundation for some research projects in operating systems. It is currently designed as a monolithic kernel plus a special kernel monitor that runs at a higher privilege level. Multiplix is very much in-progress.

The current status is that Multiplix can boot all cores, enable virtual memory and interrupts, supports multiple user-mode processes with a limited set of system calls, and has a simple Unix-like file system. Current work is focused on expanding the system call interface to support a shell and a basic user-mode environment.

Supported systems

Multiplix supports RISC-V and Armv8, specifically on the following hardware:

• VisionFive: 2-core SiFive U74 1.0 GHz.
• VisionFive 2: 4-core SiFive U74 1.25 GHz (plus a 5th SiFive S7 monitor core).
• Raspberry Pi 3: 4-core ARM Cortex A53 1.4 GHz.
• Raspberry Pi 4: 4-core ARM Cortex A72 1.5-1.8 GHz.

Support for more boards is likely to be added in the future (we have experimental versions running on the Ox64 and Orange Pi Zero 2).

Building

If you have the necessary tools, you can build and run the kernel with

knit qemu board=raspi3

See below for details:

To build multiplix you must have a GNU bare-metal toolchain and either LDC or GDC. You can get everything you need (prebuilt) from multiplix-toolchain-linux-amd64.tar.gz. You must also have Go installed to build the plboot tool (for creating bootloader payloads). You'll also need QEMU if you want to simulate the OS.

Multiplix uses the Knit build tool. The Knitfile has the following targets:

• kernel.bin: build the kernel binary.
• kernel.boot.bin: build the bootable kernel binary (kernel binary embedded in the bootloader as a payload).
• qemu: emulate the kernel using QEMU (requires qemu-system-riscv64 or qemu-system-aarch64).
• bootloader.bin: build the kernel bootloader.
• prog: send the kernel over UART to the bootloader.

You can configure the build for a specific board by specifying setting the board variable to raspi3, raspi4, visionfive, visionfive2, or virt_riscv64 (e.g., knit board=raspi3).

Specify the D compiler with dc. Supports dc=ldc or dc=gdc.

For example: knit qemu board=virt_riscv64 dc=ldc will build a kernel targeting the QEMU virt machine with LDC and run it in QEMU.

The profile option configures the optimization level and LTO. The main possible values are dev (O1 without LTO), and release (O3 with LTO). LTO with LDC requires a distribution of LDC that includes the LLVMgold linker plugin.

The unified option controls whether the build is done as a single compilation unit (one invocation of the D compiler), or in parallel with multiple compilation units.

You might also find it useful to read this blog post: https://zyedidia.github.io/blog/posts/1-d-baremetal/.

Installation on Raspberry Pi

First build the armstub firmware with

knit firmware/raspi/armstub8.bin board=raspi3

Make sure to select the correct board.

Next download the appropriate firmware:

• Raspberry Pi 3: https://www.scs.stanford.edu/~zyedidia/docs/rpi/rpi3-firmware.tar.gz
• Raspberry Pi 4: https://www.scs.stanford.edu/~zyedidia/docs/rpi/rpi4-firmware.tar.gz

Copy firmware/raspi/armstub8.bin and firmware/raspi/config.txt into the firmware folder that you downloaded.

Finally build the kernel: you can choose either the bootloader or the kernel itself. The bootloader will allow you to load new kernels over UART.

knit bootloader.bin
knit kernel.boot.bin

Copy the .bin file you choose into the firmware folder as kernel8.img. Next flash the firmware folder onto an SD card as FAT32. Finally, insert the SD card and boot up the Pi.

If you loaded the bootloader on the SD card, you can send a new kernel over UART with knit prog. Otherwise you'll want to use the rduart tool to read from the UART to view the kernel output.

Acknowledgements

Multiplix draws heavily from

• xv6 (https://github.com/mit-pdos/xv6-riscv)