[Libvirt] Attach qemu commands to an running xml domain

[Daniel P. Berrangé]

On Wed, Mar 31, 2021 at 02:11:40PM +0200, Michal Privoznik wrote:
> On 3/31/21 1:36 PM, Daniel P. Berrangé wrote:
> > 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.
> 
> Because then you'd override the path for system wide OVS. I mean, by default
> the socket is under /var/run/openvswitch/db.sock and if you have another OVS
> running inside a container you can expose it under
> /prefix/var/run/openvswitch/db.sock. ovs-vsctl allows this by --db=$path
> argument.

Oh, right I was mis-understanding the scenario.

I've always thought if we wanted to support use of resources from
other namespaces, then that would involve use of a namespace="$PID"
attribute to identify the namespace.

   <source type='unix' namespace="523532" path='/var/lib/vhost_sockets/sock7f9a971a-cf3' mode='server'/>

IOW, libvirt could enter the namespace, and then talk to the OVS
db.sock at its normal location. 

It would be  bit strange having namespace refer to a different
NS for the OVS, but using the path=/var/lib/..... from the
current namespace, but as long as we define the semantics
clearly its not the end of the world.

Regards,
Daniel
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