"The SMM, which stands for System Management Mode, is a layer that sits at the deepest level inside some types of AMD processors.

The SMM is a part of the CPU's UEFI firmware, and SMM code is usually employed to manage deep hardware-related features such as power management, system sleep, hibernations, device emulations, memory errors, and CPU safety functions.

Due to its role in keeping the CPU running and interacting with adjacent hardware components, SMM code runs with the highest level of privileges on a computer, having full control over the operating system kernel and any hypervisors (virtual machines). In technical jargon, the SMM runs on the deepest level of the CPU ring, at Ring -2."