[libvirt-users] Why does libvirt insert USB controller automagically?

[Sundar Nadathur]

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) <-- This XML string does not have a USB or a memballoon
dom = conn.defineXML(in_xml)
out_xml = dom.XMLDesc() <-- 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


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