[Qemu-devel] [Bug 1445633] [NEW] Creating a passthrough USB device causes excessive CPU usage in the guest

[P. J. Reed]

Public bug reported:

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.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1445633

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

Status in QEMU:
  New

Bug description:
  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.

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