[Libvirt] Attach qemu commands to an running xml domain

[Daniel P. Berrangé]

On Wed, Mar 31, 2021 at 01:09:32PM +0200, Michal Privoznik wrote:
> On 3/30/21 11:35 AM, Waleed Musa wrote:
> > Hi all,
> > 
> > I see in libvirt you are supporting attach/detach devices to existing
> > xml domain using *attachDeviceFlags *and *detachDeviceFlags *APIs.
> > Now we are adding some qemu command to the xml domain related to some
> > interfaces using alias names before starting the VM, but we will face an
> > issue with hot plug such devices, so I have two question here:
> > 
> >  1. Is it applicable to set the alias names for interfaces because I saw
> >     it's ignored when I add it to xml domain before starting the VM?
> >  2. Is there a way or API to attach qemu commands to running domain as
> >     you are doing in attaching the device using *attachDeviceFlags?*
> > 
> > *Example of my xml*
> > *<domain type='kvm' id='5'
> > xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
> > *
> > *  <devices>
> > *
> >      <interface type='vhostuser'>
> >        <mac address='fa:16:3e:ac:12:4c'/>
> >        <source type='unix'
> > path='/var/lib/vhost_sockets/sockbbb6bbe9-eb5' mode='server'/>
> >        <target dev='tapbbb6bbe9-eb'/>
> >        <model type='virtio'/>
> >        <driver queues='4' rx_queue_size='512' tx_queue_size='512'/>
> >        <alias name='net0'/>
> >        <address type='pci' domain='0x0000' bus='0x00' slot='0x03'
> > function='0x0'/>
> >      </interface>
> > *  </devices>
> > *
> > *  <qemu:commandline>*
> >      <qemu:arg value='-set'/>
> >      <qemu:arg value='device.net0.page-per-vq=on'/>
> >      <qemu:arg value='-set'/>
> >      <qemu:arg value='device.net0.host_mtu=8942'/>
> > *  </qemu:commandline>*
> > *</domain>*
> 
> Is this perhaps related to the following bug?
> 
> "interface type='vhostuser' libvirtError: Cannot set interface MTU"
> https://bugzilla.redhat.com/show_bug.cgi?id=1940559
> 
> Are you trying to work around it? I am discussing with Moshe how libvirt can
> help, but honestly, I don't like the solution I proposed.
> 
> Long story short, the interface is in different container (among with OVS
> bridge) and thus when we query ovs-vsctl it connects to the system one and
> doesn't find that interface. What I proposed was to allow specifying path to
> ovs db.socket but this would need to be done o per domain basis.

If it is possible to specify a ovs db.socket path in the XML, then
why can't this path simply be bind mounted to the right location
in the first place.

Regards,
Daniel
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