Hello all,

     I find this curious behavior that libvirt will automatically insert a USB controller and a memballoon virtio driver into the domain’s XML, when defineXML is called. IOW, the original XML did not have these 2 devices but they are present right after defineXML is called. Why should libvirt add a USB controller?

 

DETAILS:

Here is a little Python snippet to show that behavior:

import libvirt

 

conn = libvirt.open('qemu:///system')

xml_list = open('/tmp/vm.xml', "r").readlines()

in_xml = ''.join(xml_list) ß This XML string does not have a USB or a memballoon

dom = conn.defineXML(in_xml)

out_xml = dom.XMLDesc() ß But this XML string does!

print out_xml

 

The file /tmp/vm.xml does not have a memballoon or a USB device. Neither, of course, does in_xml string. But out_xml has:

    <controller type='usb' index='0'>

      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>

    </controller>

    <memballoon model='virtio'>

      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>

    </memballoon>

 

I don’t see anything in libvirt.conf that explains this. This is libvirt version: 1.2.8, package: 16.el7_1.5 (Red Hat, Inc....). The host is RHEL 7.1.

 

The memballoon part is understandable once I found: https://www.redhat.com/archives/libvir-list/2010-August/msg00132.html

 

However, why should libvirt add a USB controller gratuitously? Is this controllable from the conf file?

 

Cheers,

Sundar