riscv is a computer architecture just like x86 or arm, the only difference is it's open. it's simpler compared to x86 or arm. riscv is just a specification of the isa which is an abstract model that people could use to create a real cpu, or virtual machine that emulate it; so this is what I did, I made a software emulation layer that execute riscv instructions and provide io's to the programs.

1. Build

building the project is straight forward:

make

2. Run

the virtual machine is ready. the only missing part is the software. so we need a full system with a kernel and user programs to run on the vm, because the kernel will setup the environment, including io communication, and memory protection, then launches user space programs. for demo purposes you can just run the shipped in kernel:

make run

wait for the system to boot and you will get a shell.

custom image:

if you want to build your own kernel image follow these instructions:

your directory structure should look like after you setup all projects:

.
├── buildroot
├── linux
├── opensbi
├── config
├── core
├── devices
├── dts
├── helpers
├── include
├── kernel
├── main.c
├── Makefile
├── patches
├── README.md
└── soc

means all the cloning is done in the root directory.

a. rootfs

the rootfs contains the necessary programs to run in user space. we will use buildroot project for this to build an initramfs for the linux kernel.

download buildroot:

git clone --depth 1 --branch 2021.02.6 git://git.buildroot.net/buildroot buildroot

copy the config:

cp config/rv64_buildroot_config buildroot/.config

build:

(cd buildroot; make -j$(nproc))

b. Kernel

first of all we need rv64 cross compiler toolchain, checkout your distro package that has that, in my case it's archlinux:

doas pacman -S riscv64-linux-gnu-gcc

then we need to download the linux kernel source (torvalds tree):

git clone --depth 1 --branch v6.3 git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git linux

copy the config:

cp config/rv64_linux_config linux/.config

build the kernel:

(cd linux; make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- -j$(nproc))

c. opensbi

opensbi is a supervisor interface that run in machine mode and provides services for the supervisor (the kernel). it's like the firmware and it helps operating systems to access the soc services in a portable way.

download opensbi:

git clone https://github.com/riscv-software-src/opensbi.git opensbi

(cd opensbi; git checkout v1.2)

build:

(cd opensbi; make CROSS_COMPILE=riscv64-linux-gnu- PLATFORM_RISCV_XLEN=64 PLATFORM_RISCV_ISA=rv64ima_zicsr_zifencei PLATFORM=generic FW_PAYLOAD_PATH=../linux/arch/riscv/boot/Image)

d. run the image

the final image will have the following structure:

┌──────────┬───────────┬────────────────────┐
│          │           │                    │
│ opensbi  │   linux   │     buildroot      │
│          │           │                    │
└──────────┴───────────┴────────────────────┘

we also need the device tree:

make ./dts/rv64_dt.dtb

let's boot the vm:

./riscv-vm dts/rv64_dt.dtb ./opensbi/build/platform/generic/firmware/fw_payload.bin