Hi there,

I have a question about the Qos support status for different type interfaces. 
Some types of interface do not support Qos, such as hostdev, user type, mcast type, but the behavior are different, for hostdev, the guest can not start with a meaningful error message, but for other types, vm can start successfully with a warning message in the libvirtd log. I doubt that if it is necessary to keep the behavior consistent for these different types?

There are 2 history bugs for them, I should have thought further and asked early when testing the bugs. 
Bug 1319044 - log error when <bandwidth> requested on a <interface type='hostdev'>
Bug 1524230 - vhostuser type interface do not support bandwidth, but no warning message

Thank you for looking into this and very appreciate your feedback!

1. start vm with user type interface set with Qos:
 <interface type='user'>
      <mac address='52:54:00:3e:ec:14'/>
      <bandwidth>
        <inbound average='1000' peak='5000' burst='5120'/>
        <outbound average='128' peak='256' burst='256'/>
      </bandwidth>
      <model type='rtl8139'/>
      <address type='pci' domain='0x0000' bus='0x07' slot='0x01' function='0x0'/>
    </interface>

# cat /var/log/libvirt/libvirtd.log | grep bandwidth
2021-03-26 10:47:11.452+0000: 20185: warning : qemuBuildInterfaceCommandLine:8223 : setting bandwidth on interfaces of type 'user' is not implemented yet

2. start with hostdev type interface with Qos setting:
<interface type='hostdev' managed='yes'>
      <mac address='52:54:00:07:27:b0'/>
      <source>
        <address type='pci' domain='0x0000' bus='0x82' slot='0x10' function='0x6'/>
      </source>
      <bandwidth>
        <inbound average='1000' peak='5000' burst='5120'/>
        <outbound average='128' peak='256' burst='256'/>
      </bandwidth>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </interface>

# virsh start rh
error: Failed to start domain 'rh'
error: unsupported configuration: interface 52:54:00:07:27:b0 - bandwidth settings are not supported for hostdev interfaces


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Best Regards,
Yalan Zhang
IRC: yalzhang