Approaches are described for updating code and/or instructions in one or more computing devices. In particular, various embodiments provide approaches for updating the microcode of one or more processors of a computing device without requiring a restart of the computing device and without disrupting the various components (e.g., applications, virtual machines, etc.) executing on the computing device. The microcode updates can be performed on host computing devices deployed in a resource center of a service provider (e.g., cloud computing service provider), where each host computing device may be executing a hypervisor hosting multiple guest virtual machines (or other guest applications) for the customers of the service provider.