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

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?



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'/>


    <memballoon model='virtio'>

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



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?





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