1. A computer program product for measuring resource usage in a computing environment, said computer program product comprising: a non-transitory computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: selectively registering a PCI function for measurement, the selectively registering comprising executing a Modify PCI Function Controls (MPFC) instruction to register the PCI function for measurement, the PCI function being identified in the MPFC instruction by a function handle, the function handle comprising a function number identifying the PCI function, the function number configured to address the PCI function and to index into a function table that includes one or more characteristics of the PCI function, and wherein the MPFC instruction specifies a location in memory for tracking information for the PCI function; determining that the PCI function is a (a) being accessed via an instruction or (b) requesting access of system memory coupled to the PCI function; tracking information specific to the PCI function, wherein the tracked information comprises a count of determined accessing instructions or a count of determined requested accesses to memory, and wherein the tracked information is on a per-function basis; and storing the tracked information in the location in memory.