Coherency tracking apparatus and method for an attached coprocessor or accelerator

An apparatus and method for hybrid software-hardware coherency. An apparatus comprises one or more processing elements to process data; a memory controller to couple the one or more processing elements to a device memory; an interconnect to couple the one or more processing elements to a host processor memory and to couple a host processor to the device memory; one or more device caches to store cache lines read from the host processor memory and/or the device memory; coherency circuitry to manage an ownership indication for each cache line, the ownership indication to be set to a first value to indicate ownership by the host processor and to be set to a second value to indicate ownership by the processing device, wherein the coherency circuitry is to transfer ownership of a first cache line from the processing device to the host processor by updating the ownership indication from the second value to the first value, the coherency circuitry to provide indirect access to the cache line by the processing device while the ownership indication is set to the first value, the coherency circuitry to maintain the ownership indication at the first value until receiving a request to change the ownership indication.