Computer systems hardware and software designers have traditionally relied on fast emulation and full-system simulation to instrument a design of interest, develop and debug system software, model new hardware components and measure design metrics of interest. Post-Moore platforms in recent years have not only seen a proliferation of accelerators but also the need for system hardware/software co-design to help integrate heterogeneity into the system stack. Effective integration requires open-source tools that enable fast instrumentation of application and system software, full-system models for network and storage controllers, and models of multi-node computer systems. Full-system server instrumentation and modeling requires several orders of magnitude in speed to enable practical turnaround. QFlex is a family of full-system instrumentation tools based on QEMU which currently supports the ARM and RISC-V ISA. QFlex includes a trace-based model to quickly instrument existing QEMU images, and timing models to simulate multi-core CPUs in detail. For documents, papers and citations related to QFlex, please check this page.