[ovs-dev] one issue about vhost xstats with/without CRC

Yang, Zhiyong zhiyong.yang at intel.com
Wed Aug 31 09:30:58 UTC 2016


Hi, all:

Physical NIC has a set of hardware counters, such as
u64 prc64;
u64 prc127;
u64 prc255; etc.
DPDK counts the prc64 in two ways. Physical NIC counts prc64 with CRC by
hardware. Virtio computes the counter like prc64 without CRC. This will cause
the conflict, when a 64 packet from outer network is sent to VM(virtio), NIC
will show prc64 + 1, virtio will actually receive the 64-4(CRC) = 60 bytes pkt,
undersize(<64) counter will be increased, since the Length of the packet will
be subtracted CRC by hardware(NIC offload enable) or software((NIC offload
disable)).

if vhost xstats implements like NIC, It will solve the  consistency issue. But when
running vm to vm or virtio/vhost loopback test, xstats counters will count
non-existent CRC bytes .

How should Packets length of virtio/vhost be counted from OVS perspective?
With or without CRC?

Thanks
Zhiyong




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