On 3/26/21 12:31 PM, Yalan Zhang wrote:
> 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 <https://bugzilla.redhat.com/show_bug.cgi?id=1319044>- log
> error when <bandwidth> requested on a <interface type='hostdev'>
> Bug 1524230 <https://bugzilla.redhat.com/show_bug.cgi?id=1524230>-
> vhostuser type interface do not support bandwidth, but no warning message
>
> Thank you for looking into this and very appreciate your feedback!
>

The reason is historical baggage - as usual. When QoS was fist
introduced it supported only a very few interface types. Soon we've
learned that users put XML snippets in for other types too. Back then we
had no validation callbacks => we could not reject such XMLs because we
did not do it from the beginning. So there might be some domain XMLs
still that contain QoS for unsupported type and those would be lost if
libvirt started rejecting such XMLs.

With validation callbacks things are a bit better - the domain would not
be lost on libvirtd upgrade; though it would still be unable to start.
I'm not sure that's much better.

Hence, we're keeping status quo. I'm open for ideas though.

Michal