XP (crosspoint) is a switch and router logic unit and the fundamental component of the CMN interconnect. Each XP unit consists of four interconnect ports to connect to adjacent XP units and two device ports to connect to the different agents in the CHI. As shown in the figure, the CMN has six XP units configured as a 3x2 mesh network. RN-F refers to a fully coherent requesting node, HN-F refers to a fully coherent home node, and HN-D refers to the device which includes the I/O home node, DVM node, and configuration node.
RN-F and CML are assigned to common a crosspoint. Sharing a crosspoint for an RN-F/CML combination allows direct connectivity (thereby avoiding mesh traversal) for the following traffic types:
- Remote request traffic from accelerator kernel in the PL
- Remote snoop traffic from the CCIX
This benefits both latency and bandwidth between the RN-F and CML.
HN-F provides home node functionality including snoop filters. SBSX is a CHI-to-AXI bridge that allows the CMN to connect to a memory controller. HN-F and SBSX share a crosspoint. This allows direct connectivity providing latency and bandwidth benefits to the following traffic types:
- Memory traffic between HN-F and SBSX
- Snoop data from HN-F
- CHI-B direct memory transfers (where the SBSX does not transfer data to the RN-F or CML, directly bypassing HN-F)
- CHI-B and CXS data check
- CHI-B direct cache transfer (direct transfer between the CML and RN-F when either one has a dirty copy bypassing the HN-F)
- CCIX partial cache states and 128 byte cacheline support (this prevents the accelerator from using CHI-B partial opcodes)
- Address-based flush by programming CMN registers directly is not supported by the curent CMN IP version. Cache- maintenance operations (CMO) should be used instead.