>>>> we can't separate CPU and HW stats there. In some cases (or ASICs) HW >>>> counters do >>>> not include CPU generated packets....you will have to add CPU >>>> generated pkt counters to the >>>> hw counters for such virtual device stats. >>> Can you please provide and example how that could happen? >> >>example is the bridge vlan stats I mention below. These are usually counted >>by attaching hw virtual counter resources. And CPU generated packets >>in some cases maybe setup to bypass the ASIC pipeline because the CPU >>has already made the required decisions. So, they may not be counted by >>by such hw virtual counters. > > Bypass ASIC? How do the packets get on the wire? >
Bypass the "forwarding pipeline" in the ASIC that is. Obviously the ASIC ships the CPU generated packet out of the switch/front-panel port. Continuing Roopa's example of vlan netdev stats.... To get the HW stats counters are typically tied to the ingress and egress vlan hw entries. All the incoming packets are subject to the ingress vlan lookup irrespective of whether they get punted to the CPU or whether they are forwarded to another front panel port. In that case the ingress HW stats does represent all packets. However for CPU originated packets egress vlan lookups are bypassed in the ASIC (this is common forwarding option in most ASICs) and the packet shipped as is out of front-panel port specified by the CPU. Which means these packets will NOT be counted against the egress VLAN HW counter; hence the need for summation.
