X4 series adapters have two stages of packet filtering which steer traffic to different receive queues. Packets are first matched by the Fast Classifier (FC) which sends the traffic to the Express and/or Enterprise datapath. Traffic which goes via the Enterprise datapath has a further filtering stage.
Previous AMD Solarflare Ethernet adapters applied filtering in priority order. When multiple filters match the same packet, the most specific filter match would always be used. In X4 series adapters, the two stage filtering makes this more complex to achieve. By default, the X4 firmware uses a “filter tracking” feature, which ensures the X4 applies the filters in priority order.