1. A computer system, comprising: a virtual machine (VM) configuration comprising one or more cores, each core enabled to operate in a single thread (ST) mode or a multithreading (MT) mode, the ST mode consisting of single thread and the MT mode consisting of a plurality of threads on shared resources of a respective core; a core-oriented system control area (COSCA) comprising a common area representing all of the one or more cores of the VM configuration and separate core description areas for each core of the one or more cores in the VM configuration, wherein each core description area indicates a location of one or more thread description areas each representing a thread within the respective core, and each thread description area indicates a location of a state description of the respective thread; and a facility configured to control the configuration to perform a method comprising: managing threads of the one or more cores of the VM configuration based on contents of the COSCA.