[dpdk-dev] [RFC PATCH] ethdev: clarify flow action PORT ID semantics

[Thomas Monjalon]

01/06/2021 14:10, Ilya Maximets:
> On 6/1/21 1:14 PM, Ivan Malov wrote:
> > By its very name, action PORT_ID means that packets hit an ethdev with the
> > given DPDK port ID. At least the current comments don't state the opposite.
> > That said, since port representors had been adopted, applications like OvS
> > have been misusing the action. They misread its purpose as sending packets
> > to the opposite end of the "wire" plugged to the given ethdev, for example,
> > redirecting packets to the VF itself rather than to its representor ethdev.
> > Another example: OvS relies on this action with the admin PF's ethdev port
> > ID specified in it in order to send offloaded packets to the physical port.
> > 
> > Since there might be applications which use this action in its valid sense,
> > one can't just change the documentation to greenlight the opposite meaning.
> > This patch adds an explicit bit to the action configuration which will let
> > applications, depending on their needs, leverage the two meanings properly.
> > Applications like OvS, as well as PMDs, will have to be corrected when the
> > patch has been applied. But the improved clarity of the action is worth it.
> > 
> > The proposed change is not the only option. One could avoid changes in OvS
> > and PMDs if the new configuration field had the opposite meaning, with the
> > action itself meaning delivery to the represented port and not to DPDK one.
> > Alternatively, one could define a brand new action with the said behaviour.
> 
> We had already very similar discussions regarding the understanding of what
> the representor really is from the DPDK API's point of view, and the last
> time, IIUC, it was concluded by a tech. board that representor should be
> a "ghost of a VF", i.e. DPDK APIs should apply configuration by default to
> VF and not to the representor device:
>   https://patches.dpdk.org/project/dpdk/cover/20191029185051.32203-1-thomas@monjalon.net/#104376
> This wasn't enforced though, IIUC, for existing code and semantics is still mixed.

Quoting myself from above link:
"the representor port must be a real DPDK port, not a ghost."
and
"During the Technical Board yesterday, it was decided to go with Intel
understanding of what is a representor, i.e. a ghost of the VF."
and
"we will continue to mix VF and representor operations
with the same port ID. For the record, I believe it is very bad."

> I still think that configuration should be applied to VF, and the same applies
> to rte_flow API.  IMHO, average application should not care if device is
> a VF itself or its representor.  Everything should work exactly the same.

What means "work exactly the same"?
Is it considering what is behind the representor silently,
or considering the representor as a real port?

There is a need to really consider representor port as any other port,
and stop this ugly mix. I want to propose such change again for DPDK 21.11.
To me the real solution is to use a bit in the port id of a representor
for explicitly identifying the port behind the representor.
This bit could be translated as a flag or a sign in testpmd text grammar.

> I think this matches with the original idea/design of the switchdev functionality
> in the linux kernel and also matches with how the average user thinks about
> representor devices.

There is no "average" user or application, just right and wrong.
In the switchdev model, a representor is a port of a switch like
any other port, not a ghost of its peer.

> If some specific use-case requires to distinguish VF from the representor,
> there should probably be a separate special API/flag for that.

Yes, port ID of a representor must be the representor itself,
and a bit can help reaching the port behind the representor.