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

[Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>]

On 6/1/21 5:44 PM, Eli Britstein wrote:
> 
> On 6/1/2021 5:35 PM, Andrew Rybchenko wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> On 6/1/21 4:24 PM, Eli Britstein wrote:
>>> On 6/1/2021 3:10 PM, Ilya Maximets wrote:
>>>> External email: Use caution opening links or attachments
>>>>
>>>>
>>>> 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.
>>> It doesn't make any sense to attach the VF itself to OVS, but only its
>>> representor.
>> OvS is not the only DPDK application.
> True. It is just the focus of this commit message is OVS.
>>
>>> For the PF, when in switchdev mode, it is the "uplink representor", so
>>> it is also a representor.
>> Strictly speaking it is not a representor from DPDK point of
>> view. E.g. representors have corresponding flag set which is
>> definitely clear in the case of PF.
> This is the per-PMD responsibility. The API should not care.
>>
>>> That said, OVS does not care of the type of the port. It doesn't matter
>>> if it's an "upstream" or not, or if it's a representor or not.
>> Yes, it is clear, but let's put OvS aside. Let's consider a
>> DPDK application which has a number of ethdev port. Some may
>> belong to single switch domain, some may be from different
>> switch domains (i.e. different NICs). Can I use PORT_ID action
>> to redirect ingress traffic to a specified ethdev port using
>> PORT_ID action? It looks like no, but IMHO it is the definition
>> of the PORT_ID action.
> 
> Let's separate API from implementation. By API point of view, yes, the
> user may request it. Nothing wrong with it.
> 
> From implementation point of view - yes, it might fail, but not for
> sure, even if on different NICs. Maybe the HW of a certain vendor has
> the capability to do it?
> 
> We can't know, so I think the API should allow it.

Hold on. What should it allow? It is two opposite meanings:
 1. Direct traffic to DPDK ethdev port specified using ID to be
    received and processed by the DPDK application.
 2. Direct traffic to an upstream port represented by the
    DPDK port.

The patch tries to address the ambiguity, misuse it in OvS
(from my point of view in accordance with the action
documentation), mis-implementation in a number of PMDs
(to work in OvS) and tries to sort it out with an explanation
why proposed direction is chosen. I realize that it could be
painful, but IMHO it is the best option here. Yes, it is a
point to discuss.

To start with we should agree that that problem exists.
Second, we should agree on direction how to solve it.

>>
>>>> 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.
>>> I am not sure how this is related.
>>>> 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.
>>>> 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.
>>> Right. This is the way representors work. It is fully aligned with
>>> configuration of OVS-kernel.
>>>> If some specific use-case requires to distinguish VF from the
>>>> representor,
>>>> there should probably be a separate special API/flag for that.
>>>>
>>>> Best regards, Ilya Maximets.