This is the step 4 of the book Writing a RISC-V Emulator from Scratch in 10 Steps, whose goal is running xv6, a small Unix-like OS, in your emulator in the final step.
The source code is available at d0iasm/rvemu-for-book/step4/.
In the end of this page, we can execute parts of supervisor ISA, mret, and sret. These instructions are used to return from traps in M-mode, S-mode, or U-mode respectively.