Modern CPU's already have several counters for 'nominal' cycles (i.e. cycles at a constant frequency). e.g. TSC instructions and Performance-Monitoring Counters

Modern CPU's already have several counters for 'nominal' cycles (i.e. cycles at a constant frequency). e.g. TSC instructions and Performance-Monitoring Counters. Converting nominal cycles to time can be unreliable in some cases but not impossible.