riscv64i #[no_std] Linux user space example

This repository serves to my later self as reference to document some #[no_std] and inline assembly features as well as cargo configurations.

It builds a riscv64i Linux user space binary which invokes some syscalls according to the Linux syscall(2) ABI for riscv.

Arch/ABI    Instruction           System  Ret  Ret  Error    Notes
                                  call #  val  val2
───────────────────────────────────────────────────────────────────
riscv       ecall                 a7      a0   a1   -

Arch/ABI      arg1  arg2  arg3  arg4  arg5  arg6  arg7  Notes
──────────────────────────────────────────────────────────────
riscv         a0    a1    a2    a3    a4    a5    -

The syscalls are implemented in lib.rs.

When building, cargo runs the build.rs script before building the crate itself, which in this case generates some rust code for the syscall numbers from the riscv C headers.

Looking into .cargo/config we actually build for the riscv64imac-unknown-none-elf rather than the riscv64gc-unknown-linux-gnu target, as our goal is to build a risv64*i* binary and that way we just have to disable a few more extensions :^).

The binary can be automatically run in the QEMU user space emulator with cargo run, this is because the runner is specified in .cargo/config.

Requirements

• riscv64-linux-gnu-gcc toolchain for parsing syscall numbers in build.rs
• riscv64imac-unknown-none-elf rustc target to generate riscv target code (rustup target add riscv64imac-unknown-none-elf)
• qemu-riscv64 QEMU userspace emulator