Techniques are disclosed for maintaining cache coherency across a serial interface bus such as a Peripheral Component Interconnect Express (PCIe) bus. The techniques include generating a snoop request (SNP) to determine whether first data stored in a local memory is coherent relative to second data stored in a data cache, the snoop request including destination information that identifies the data cache on the serial interface bus and causing the snoop request to be transmitted over the serial interface bus to a second processor. The techniques further include extracting a cache line address from the snoop request, determining whether the second data is coherent, generating a complete message (CPL) indicating that the first data is coherent with the second data, and causing the complete message to be transmitted over the bus to the first processor. The snoop request and complete messages may be vendor defined messages.