netvsc: There are always too many packets in tx-drop queue in testpmd tx-side

https://bugs.dpdk.org/show_bug.cgi?id=390

Bug ID: 390
Summary: netvsc: There are always too many packets in tx-drop
queue in testpmd tx-side
Product: DPDK
Version: 18.11
Hardware: x86
OS: Linux
Status: UNCONFIRMED
Severity: normal
Priority: Normal
Component: ethdev
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---

Description of problem:
While we test dpdk on Azure with testpmd, there are too many packets in tx-drop queue.

Version-Release number of selected component (if applicable):
DPDK-18.11

How reproducible:
100%

Steps to Reproduce:
1.Prepare 2 vms on Azure with dpdk installed
2.The 1st vm, start testpmd with txonly mode. The 2nd one start testpmd with rx-only mode.And set 1st vm's peer mac address as the 2nd vm's mac address. Conversely, set 2nd vm's peer mac address as the 1st one's mac address.

#TX-SIDE
#./testpmd -c 0xf -n 1 -w 0002:00:02.0 --vdev="net_vdev_netvsc0,iface=eth1,force=1" -- --port-topology=chained --nb-cores 1 --forward-mode=txonly --eth-peer=1,00:0d:3a:a1:4b:43 --stats-period 1

#RX-SIDE
#./testpmd -c 0xf -n 1 -w 0002:00:02.0 --vdev="net_vdev_netvsc0,iface=eth1,force=1" -- --port-topology=chained --nb-cores 1 --forward-mode=rxonly --eth-peer=1,00:0d:3a:a1:41:c7 --stats-period 1

3.Keep testpmd running up to 1 minute,then stop testpmd.

4.Check 2 vms' output

Actual results:
tx-side
++++++++ Accumulated forward statistics for all ports++++++++++
RX-packets: 0 RX-dropped: 0 RX-total: 0
TX-packets: 340381695 TX-dropped: 241822721 TX-total: 582204416
++++++++++++++++++++++++++++++++++++++++++++++
rx-side
++++++++ Accumulated forward statistics for all ports++++++
RX-packets: 339493162 RX-dropped: 0 RX-total: 339493162
TX-packets: 0 TX-dropped: 0 TX-total: 0
+++++++++++++++++++++++++++++++++++++++++++++

Expected results:
There shouldn't be too many(40%-50%) packets in TX-dropped queue

Additional info:
The bug occurs in latest DPDK upstream

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