libvirt: how to pass qemu command-line args now libvirt schemas is 404

Question

This is the only way I have found to pass arguments to qemu:

<domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>

Unfortunately the url http://libvirt.org/schemas/domain/qemu/1.0 now simply returns a 404 page, and libvirt automatically trims this part off. I can't seem to find any information on an alternative as even the libvirt website advises adding this broken schema!

Is there is another way to enable evdev pass-though without qemu arguments?

EDIT:

I solved this using the solution marked below, but one important thing to note is that virsh will always trim off xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0' unless there is a valid <qemu:[whatever]> tag somewhere in the domain.

No, that post is outdated, the virsh import functionality has since been deprecated and does not work for this purpose, another solution posted there is exactly where I run into the problem mentioned above. — Alex Baldwin, Jun 06 '20 at 21:49

score 3 · Accepted Answer · edited Jun 08 '20 at 08:36

Even if the URL of the namespace is a 404, using it in a domain configuration does work, and is documented on the libvirt homepage:

Additionally, the following XML additions allow fine-tuning of the command line given to qemu when starting a domain (Since 0.8.3). In order to use the XML additions, it is necessary to issue an XML namespace request (the special xmlns:name attribute) that pulls in http://libvirt.org/schemas/domain/qemu/1.0; typically, the namespace is given the name of qemu. With the namespace in place, it is then possible to add an element <qemu:commandline> under domain, with the following sub-elements repeated as often as needed:

qemu:arg

Add an additional command-line argument to the qemu process when starting the domain, given by the value of the attribute value.

qemu:env

Add an additional environment variable to the qemu process when starting the domain, given with the name-value pair recorded in the attributes name and optional value.

This is also tested in various places in libvirt:

I tested this solution on a kvm domain by changing the beginning of the definition to

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <qemu:commandline>
    <qemu:arg value='-unknown'/>
    <qemu:arg value='parameter'/>
  </qemu:commandline>

in virsh -c qemu:///system edit <domain>. On subsequent edits, those changes still exist (the new elements are moved to the end of the document, though).

Starting the domain also fails with qemu complaining about an unknown parameter:

↪ virsh -c qemu:///system start <domain>
error: Failed to start domain <domain>
error: Interner Fehler: qemu unexpectedly closed the monitor: qemu-system-x86_64: -unknown: invalid option

Note that an XML namespace does not need to be a URL that resolves to something ... usable.

libvirt: how to pass qemu command-line args now libvirt schemas is 404

1 Answers1