libvirt users - Sep 2022 - Why host device disappear in libvirt doman xml?

<https://stackoverflow.com/posts/73854544/timeline>

I'm using Kubevirt to manage my virtual machine instances. When I using
Kubevirt to create a vm(with two GPUs), kubevirt will generate a libvirt
guest domain xml for this vm which includes two GPUs, the domain xml as
follow :

    <hostdev mode='subsystem' type='pci'
managed='no'>
      <driver name='vfio'/>
      <source>
        <address domain='0x0000' bus='0x83'
slot='0x00' function='0x0'/>
      </source>
      <alias name='ua-gpu-gpu0'/>
      <address type='pci' domain='0x0000' bus='0x06'
slot='0x00'
function='0x0'/>
    </hostdev>
    <hostdev mode='subsystem' type='pci'
managed='no'>
      <driver name='vfio'/>
      <source>
        <address domain='0x0000' bus='0x84'
slot='0x00' function='0x0'/>
      </source>
      <alias name='ua-gpu-gpu1'/>
      <address type='pci' domain='0x0000' bus='0x07'
slot='0x00'
function='0x0'/>
    </hostdev>

No one ever edit this domain xml, but these two hostdev element disappeared
in the domain xml. During this time, I've run the gpu_burn command to do a
stress test for these two GPUs.

My question is :

   - when will libvirtd change the guest domain xml ?
   - why libvirtd delete these two hostdev from domain xml ?
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On 9/26/22 15:06, ??? wrote:
> 
> <https://stackoverflow.com/posts/73854544/timeline>
> 
> I'm using Kubevirt to manage my virtual machine instances. When I using
> Kubevirt to create a vm(with two GPUs), kubevirt will generate a libvirt
> guest domain xml for this vm which includes two GPUs, the domain xml as
> follow :
> 
> |<hostdev mode='subsystem' type='pci'
managed='no'> <driver
> name='vfio'/> <source> <address domain='0x0000'
bus='0x83' slot='0x00'
> function='0x0'/> </source> <alias
name='ua-gpu-gpu0'/> <address
> type='pci' domain='0x0000' bus='0x06'
slot='0x00' function='0x0'/>
> </hostdev> <hostdev mode='subsystem' type='pci'
managed='no'> <driver
> name='vfio'/> <source> <address domain='0x0000'
bus='0x84' slot='0x00'
> function='0x0'/> </source> <alias
name='ua-gpu-gpu1'/> <address
> type='pci' domain='0x0000' bus='0x07'
slot='0x00' function='0x0'/>
> </hostdev> |
> 
> No one ever edit this domain xml, but these two?|hostdev|?element
> disappeared in the domain xml. During this time, I've run
> the?|gpu_burn|?command to do a stress test for these two GPUs.
> 
> My question is :
> 
>   * when will libvirtd change the guest domain xml ?
>   * why libvirtd delete these two?|hostdev|?from domain xml ?
> 
Libvirt does not remove anything from domain XML (except for the
elements it does not understand, but this is not the case). My suspicion
is that you're looking at live XML instead of inactive XML or vice
versa. Libvirt allows guests to be defined (i.e. libvirt manages their
inactive definition). However, a guest can be started with wildly
different configuration (e.g. without those two <hostdev/>-s). OR, they
might have been hot-unplugged.


This article can explain more details:
https://wiki.libvirt.org/page/VM_lifecycle

Michal