[Libvirt] Attach qemu commands to an running xml domain

[Peter Krempa]

On Tue, Mar 30, 2021 at 09:35:48 +0000, 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?

User specified aliases are possible with 'ua-' prefix:

https://www.libvirt.org/formatdomain.html#devices


>   2.  Is there a way or API to attach qemu commands to running domain as you are doing in attaching the device using  attachDeviceFlags?

No. For VMs which use <qemu:commandline> we don't formally provide
support. I suggest that if you have a good use case for the attributes
below, you send patches adding the functionality to libvirt if you want
to have it on device hotplug.

> 
> 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>
> 
> 
> Regards,
> Waleed Mousa
> Software Engineer,
> Nvidia <https://www.nvidia.com/en-me/geforce/>
>