[OVN] min-BW rule, defined in the LSP.options field, must be accompanied with a max-BW value

[Rodolfo Alonso]

The QoS values defined in the LSP.options field are directly translated to a TC command and set in the interface. There are 3 values [1]:
* qos_min_rate
* qos_max_rate
* qos_burst

Neutron is currently using "qos_min_rate" only. The max-BW and DSCP QoS rules are defined with QoS registers that match an input/output port traffic.

As specified in [2], the "rate" parameter ("qos_max_rate") is mandatory. The "ceil" parameter ("qos_min_rate") is not. Neutron needs to provide a default "qos_max_rate" for any port where "qos_min_rate" is configured (as commented, we are only using "qos_min_rate").

[1]https://github.com/ovn-org/ovn/blob/1bec9e3ddd8500793b52e11c3dc1f41ef1f48591/ovn-nb.xml#L1155-L1168
[2]https://man7.org/linux/man-pages/man8/tc-htb.8.html

[Bence Romsics (bence-romsics)]

Don't we have the relationships the other way around?

guaranteed bw - qos_min_rate in ovn - rate in tc
shaped bw - qos_max_rate in ovn - ceil in tc

But anyway I see your point. This must break setting ceil without rate. But while broken I guess this can be worked around by providing both in the API.

In a fix we can provide a minimal (0 if allowed) rate when there's no user input coming from the API.

[Rodolfo Alonso (rodolfo-alonso-hernandez)]

Hi Bence:

The "tc" [1] defines only "rate" as mandatory (disregard the "burst" parameter, check the parameter description down in this page). That means:
* If we define max-bw, only "qos_max_rate" is needed (and maybe burst)
* If we define min-bw, we need "qos_min_rate" and a value of "qos_max_rate" ("qos_max_rate" >= "qos_min_rate" always).

In any case, in Neutron we only use "LSP.options.qos_xxx_rate" for min-bw rules. For max-bw and DSCP we use QoS registers that filter the traffic depending on the ingress/egress port and apply the needed traffic shaping (actually policing). This is why we can't fix that providing max-bw in the API, this rule will generate a OVN QoS register but won't populate the "LSP.options.qos_max_rate".

In this case we should provide a default "LSP.options.qos_max_rate" value, big enough not to interfere with the traffic egressing the interface.

Regards.

[1]https://man7.org/linux/man-pages/man8/tc-htb.8.html