In a logically partitioned host computer system comprising host processors (host CPUs), a facility and instruction for discovering topology of one or more guest processors (guest CPUs) of a guest configuration comprises a guest processor of the guest configuration fetching and executing a STORE SYSTEM INFORMATION instruction that obtains topology information of the computer configuration. The topology information comprising nesting information of processors of the configuration and the degree of dedication a host processor provides to a corresponding guest processor. The information is preferably stored in a single table in memory.