There's things that can be set by writing to MSRs that are not able to be unset until the processor is physically reset. An example is the option that enables/

There's things that can be set by writing to MSRs that are not able to be unset until the processor is physically reset. An example is the option that enables/disables the whole VMX/SVM capability - and that is something the firmware will typically do according to an option in NVRAM early in the boot process. I think SMM lock falls under the same umbrella.