1. A computer-implemented method, comprising: under the control of one or more computer systems configured with executable instructions, receiving an application programming interface request for a secure execution environment; and fulfilling the application programming interface request by at least: selecting a target computer system from a plurality of target computer systems, the target computer system selected based at least in part on the target computer system being operable to instantiate the secure execution environment; sending a provisioning request to the target computer system to cause the secure execution environment to be instantiated on a processor of the target computer system; and validating the secure execution environment using one or more cryptographic measurements calculated by the processor.