You may find the details of the implementation here: https://adamkostrzewa.github.io/jekyll/update/2018/05/31/sparc-qemu-trojan.html
This is a short tutorial for implementation of the CPU backdoor with the help of QEMU emulator.
Requirements:
- QEMU emulator version 2.12.50 (v2.12.0-rc3-71-g6af2692e86-dirty) cloned from git clone git://git.qemu.org/qemu.git
- buildroot-2018.02.1 for cross compilation cloned from git clone git://git.buildroot.net/buildroot