vesper

A small RISC-V 64 kernel written in Rust, following Stephen Marz's osblog tutorial.

Current Status

Implemented so far:

• M-mode boot path (_start) with PMP setup and handoff to Rust
• Sv39 paging with identity mapping of kernel memory and MMIO regions
• M-mode trap handling (timer + external + syscall path)
• PLIC + UART interrupt-driven console input
• Simple round-robin kernel-thread scheduler
• VirtIO legacy MMIO probe + VirtIO block driver (Chapter 9 scope)
• Boot-time block read smoke test from hdd.dsk
• Minix 3 filesystem reader: superblock, inodes, direct/indirect zone walk, directory entries (Chapter 10 scope)
• Syscall #63 (read(dev, inode, buf, size, offset)) routed through the Minix 3 reader
• ELF64 header + program-header parser, RISC-V/EXEC validation, and a boot-time test_elf path that loads + dumps a binary by inode (Chapter 11 scope)

Known gaps relative to Chapter 11: the ELF loader parses and validates binaries, but actually executing them in U-mode requires per-process satp switching in the M-mode trap vector — that wiring is not in yet, so the demo currently only summarises program headers rather than mret-ing into the loaded entry point.

Toolchain

rustup target add riscv64gc-unknown-none-elf

The crate uses Rust 2024 edition (Rust 1.85+).

QEMU

You need qemu-system-riscv64:

# Debian / Ubuntu
sudo apt install qemu-system-misc
# Arch / Manjaro
sudo pacman -S qemu-system-riscv
# macOS
brew install qemu

Build

cargo build              # debug
cargo build --release    # optimised

The target and -T linker flag are baked into .cargo/config.toml, so a plain cargo build cross-compiles to RISC-V automatically.

Disk Image

The QEMU runner is configured with a virtio-blk-device backed by hdd.dsk. The runner in .cargo/config.toml auto-creates hdd.dsk on first cargo run: it allocates a 32 MiB image and, if mkfs.minix is on PATH, formats it as a Minix 3 filesystem so the chapter-10 reader can mount it on boot.

Run

cargo run

That invokes the runner in .cargo/config.toml:

qemu-system-riscv64 \
    -machine virt -cpu rv64 -smp 4 -m 128M \
    -display none -serial stdio \
    -bios none \
    -drive file=hdd.dsk,if=none,format=raw,id=hd0 \
    -device virtio-blk-device,drive=hd0 \
    -kernel target/riscv64gc-unknown-none-elf/debug/vesper