Apparatus, methods, and computer-readable storage media are disclosed for core-to-core communication between physical and/or virtual processor cores. In some examples of the disclosed technology, application cores write notification data (e.g., to doorbell or PCI configuration memory space accesses via a memory interface), without synchronizing with the other application cores or the service cores. In one examples of the disclosed technology, a message selection circuit is configured to, serialize data from the plurality of user cores by: receiving data from a user core, selecting one of the service cores to send the data based on a memory location addressed by the sending user core, and sending the received data to a respective message buffer dedicated to the selected service core.