Re: [libvirt-users] Why does libvirt insert USB controller automagically?
On 11/30/2015 07:30 PM, Sundar Nadathur wrote:
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?
Historical compatibility. Older qemu would provide these devices automatically
with no way for libvirt to disable them. Eventually qemu grew to support
disabling these devices, but libvirt qemu users were already expecting them to
be added automatically, so we are stuck with that API contract.
If you want to disable them, you have to explicitly add
<controller type='usb' model='none'/>
<memballoon model='none'/>
- Cole
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
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