Qemu/KVM: packet queueing from/to guest


I am new to virtualization and running into a problem when packets are sent out at higher rate from guest running on kvm based host. I am seeing ‘overruns’ counters increment for a vnetXX interface. To circumvent this issue I increased txqueuelen from default value of 500 to 10k, this solves the issue, but I would like to understand the root cause of the issue, is it happening because of the slow draining from guest or qemu is not able to drain out packets from guest. Essentially I would like to understand in which datapath (from guest or to guest) txqueuelen parameter is being used.

some info about my environment
qemu net creation options
-netdev tap,fd=25,id=hostnet0,vhost=on,vhostfd=26 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=fa:ac:00:00:01:01,bus=pci.0,addr=0x3,bootindex=1

Note: though vhost=on but it is not using vhost driver, since guest doesn’t support msi-x

and type of vnet

ethtool -i vnet0
driver: tun
version: 1.6
firmware-version: N/A
bus-info: tap
supports-statistics: no

Any help or pointers would be highly appreciated.