Abstract: GenMC is an LLVM-based state-of-the-art stateless model checker for concurrent C/C++ programs. Its modular infrastructure allows it to support complex memory models, such as RC11 and IMM, and makes it easy to extend to support further axiomatic memory models. In this paper, we discuss the overall architecture of the tool and how it can be extended to support additional memory models, programming languages, and/or synchronization primitives. To demonstrate the point, we have extended the tool with support for the Linux kernel memory model (LKMM), synchronization barriers, POSIX I/O system calls, and better error detection capabilities.