Creating a passthrough USB device causes excessive CPU usage in the guest

[P. J. Reed]

My host machine is a Lenovo X1 Carbon (3rd gen) laptop running 64-bit Ubuntu Linux 14.04. My guest machine is running 64-bit Ubuntu Linux 12.04. My QEMU was compiled moments ago from the git repository, and it says its version is:
qemu-x86_64 version 2.2.93, Copyright (c) 2003-2008 Fabrice Bellard

My issue is that when I create a passthrough for a USB host device to the guest, it causes qemu's CPU utilization on the host to increase significantly and stay there. After the passthrough is created, qemu's CPU usage in top hovers between 20% and 30% (evenly spread across CPUs). In virt-manager's CPU usage chart, it shows the guest's CPU usage growing from 0% to about 5%, although running "top" in the guest does not show an increase in CPU usage. These usage levels drop back down to normal (0 to 1%) only after I physically unplug the device or remove the passthrough mapping.

It doesn't seem to matter what the device is. I have tested it using:
A Realtek rtl8192cu-based 802.11n adapter
An HP optical mouse
A Samsung Galaxy S5 phone

The behavior is the same regardless of device, and they otherwise seem to work properly in the guest machine.

[Thomas Huth (th-huth)]

Looking through old bug tickets... can you still reproduce this issue with the latest version of QEMU? Or could we close this ticket nowadays?

[Launchpad Janitor (janitor)]

[Expired for QEMU because there has been no activity for 60 days.]