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

[Ilya Maximets <i.maximets@ovn.org>]

On 6/1/21 4:28 PM, Ivan Malov wrote:
> Hi Ilya,
> 
> Thank you for reviewing the proposal at such short notice. I'm afraid that prior discussions overlook the simple fact that the whole problem is not limited to just VF representors. Action PORT_ID is also used with respect to the admin PF's ethdev, which "represents itself" (and by no means it represents the underlying physical/network port). In this case, one cannot state that the application treats it as a physical port, just like one states that the application perceives representors as VFs themselves.


I don't think that it was overlooked.  If device is in a switchdev mode than
there is a PF representor and VF representors.  Application typically works
only with representors in this case is it doesn't make much sense to have
representor and the upstream port attached to the same application at the
same time.  Configuration that is applied by application to the representor
(PF or VF, it doesn't matter) applies to the corresponding upstream port
(actual PF or VF) by default.

Exactly same thing here with PORT_ID action.  You have a packet and action
to send it to the port, but it's not specified if HW needs to send it to
the representor or the upstream port (again, VF or PF, it doesn't matter).
Since there is no extra information, HW should send it to the upstream
port by default.  The same as configuration applies by default to the
upstream port.

Let's look at some workflow examples:

      DPDK Application
        |         |
        |         |
   +--PF-rep------VF-rep---+
   |                       |
   |    NIC (switchdev)    |
   |                       |
   +---PF---------VF-------+
       |          |
       |          |
   External       VM or whatever
   Network

a. Workflow for "DPDK Application" to set MAC to VF:

1. "DPDK Application" calls rte_set_etheraddr("VF-rep", new_mac);
2.  DPDK sets MAC for "VF".

b. Workflow for "DPDK Application" to set MAC to PF:

1. "DPDK Application" calls rte_set_etheraddr("PF-rep", new_mac);
2.  DPDK sets MAC for "PF".

c. Workflow for "DPDK Application" to send packet to the external network:

1. "DPDK Application" calls rte_eth_tx_burst("PF-rep", packet);
2. NIC receives the packet from "PF-rep" and sends it to "PF".
3. packet egresses to the external network from "PF".

d. Workflow for "DPDK Application" to send packet to the "VM or whatever":

1. "DPDK Application" calls rte_eth_tx_burst("VF-rep", packet);
2. NIC receives the packet from "VF-rep" and sends it to "VF".
3. "VM or whatever" receives the packet from "VF".

In two workflows above there is no rte_flow processing on step 2, i.e.,
NIC does not perform any lookups/matches/actions, because it's not possible
to configure actions for packets received from "PF-rep" or
"VF-rep" as these ports doesn't own a port id and all the configuration
and rte_flow actions translated and applied for the devices that these
ports represents ("PF" and "VF") and not representors themselves ("PF-rep"
or "VF-rep").

e. Workflow for the packet received on PF and PORT_ID action:

1. "DPDK Application" configures rte_flow for all packets from "PF-rep"
   to execute PORT_ID "VF-rep".
2. NIC receives packet on "PF".
3. NIC executes 'PORT_ID "VF-rep"' action by sending packet to "VF".
4. "VM or whatever" receives the packet from "VF".

f. Workflow for the packet received on VF and PORT_ID action:

1. "DPDK Application" configures rte_flow for all packets from "VF-rep"
   to execute 'PORT_ID "PF-rep"'.
2. NIC receives packet on "VF".
3. NIC executes 'PORT_ID "PF-rep"' action by sending packet to "PF".
4. Packet egresses from the "PF" to the external network.

Above is what, IMHO, the logic should look like and this matches with
the overall switchdev design in kernel.

I understand that this logic could seem flipped-over from the HW point
of view, but it's perfectly logical from the user's perspective, because
user should not care if the application works with representors or
some real devices.  If application configures that all packets from port
A should be sent to port B, user will expect that these packets will
egress from port B once received from port A.  That will be highly
inconvenient if the packet will ingress from port B back to the
application instead.

      DPDK Application
        |          |
        |          |
     port A     port B
        |          |
      *****MAGIC*****
        |          |
   External      Another Network
   Network       or VM or whatever

It should not matter if there is an extra layer between ports A and B
and the external network and VM.  Everything should work in exactly the
same way, transparently for the application.

The point of hardware offloading, and therefore rte_flow API, is to take
what user does in software and make this "magically" work in hardware in
the exactly same way.  And this will be broken if user will have to
use different logic based on the mode the hardware works in, i.e. based on
the fact if the application works with ports or their representors.

If some specific use case requires application to know if it's an
upstream port or the representor and demystify the internals of the switchdev
NIC, there should be a different port id for the representor itself that
could be used in all DPDK APIs including rte_flow API or a special bit for
that matter.  IIRC, there was an idea to add a bit directly to the port_id
for that purpose that will flip over behavior in all the workflow scenarios
that I described above.

> 
> Given these facts, it would not be quite right to just align the documentation with the de-facto action meaning assumed by OvS.

It's not a "meaning assumed by OvS", it's the original design and the
main idea of a switchdev based on a common sense.

> 
> On 01/06/2021 15:10, Ilya Maximets wrote:
>> 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.
>>
>> 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.
>>
>> 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.
>>
>