Passing QEMU command line options through libvirt

This one comes from Stefan's blog with some duct tape and bailing wire courtesy of Laine.  I've talked previously about using wrapper scripts to launch QEMU, which typically use sed to insert options that libvirt doesn't know about.  This is by far better than defining full vfio-pci devices using <qemu:arg> options, which many guides suggest, but it hides the devices from libvirt and causes all sorts of problems with device permissions and locked memory, etc.  But, there's a nice compromise as Stefan shows in his last example at the link above.  Say we only want to add x-vga=on to one of our hostdev entries in the libvirt domain XML.  We can do something like this:

<qemu:commandline>

  <qemu:arg value='-set'/>

  <qemu:arg value='device.hostdev0.x-vga=on'/>

</qemu:commandline>

The effect is that we add the option x-vga=on to the hostdev0 device, which is defined via a normal <hostdev> section in the XML and gets all the device permission and locked memory love from libvirt.  So which device is hostdev0?  Well, things get a little mushy there.  libvirt invents the names based on the order of the hostdev entries in the XML, so you can simply count (starting from zero) to pick the entry for the additional option.  It's a bit cleaner overall than needing to manage a wrapper script separately from the VM.  Also, don't forget that to use <qemu:commandline> you need to first enable the QEMU namespace in the XML by updating the first line in the domain XML to:

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

Otherwise libvirt will promptly discard the extra options when you save the domain.