NERV is a very simple single-stage RV32I processor. It is equipped with an RVFI interface and is formally verified.
git clone https://github.com/yosyshq/nerv.git
cd nerv
make
git clone https://github.com/yosyshq/riscv-formal.git
cd riscv-formal/cores/
git clone https://github.com/yosyshq/nerv.git
cd nerv
make -j8 check
See the iCEBreaker SOC README