Re: [dpdk-dev] [PATCH v4 0/5] lib: add Port Representors

["Doherty, Declan" <declan.doherty@intel.com>]

On 09/01/2018 11:22 PM, Thomas Monjalon wrote:
> Hi,
> 
> 08/01/2018 15:37, Remy Horton:
>> Port Representors provide a logical presentation in DPDK of VF (virtual
>> function) ports for the purposes of control and monitoring. Each port
>> representor device represents a single VF and is associated with it's
>> parent physical function (PF) PMD which provides the back-end hooks for
>> the representor device ops and defines the control domain to which that
>> port belongs. This allows to use existing DPDK APIs to monitor and control
>> the port without the need to create and maintain VF specific APIs.
> 
> Extending control plane ability of DPDK has been discussed
> multiple times.

It has, and I have yet to see a really strong reason as to why we would 
not support control plane functions within DPDK, many of which are 
already support today implicitly anyway through our ethdev APIs.

> The current agreed policy is:
> "
> The primary goal of DPDK is to provide a userspace dataplane.
> Managing VFs from a PF driver is a control plane feature and developers
> should generally rely on the Linux Kernel for that.
> "
> http://dpdk.org/doc/guides/contributing/design.html#pf-and-vf-considerations
> 

My understanding is that this particular entry was based around the
discussion on the divergence of functionality between the Linux kernel
PF driver and the DPDK PF driver. I also don't really think the above
statement is valid as a blanket statement for the project as it makes 
the assumption that DPDK is only deployed on Linux hosts, what about 
FreeBSD? or in the future Windows?

A number of presentations at both Userspace in Dublin and the Summit
in San Jose discussed the support of control plane functionality by
DPDK and there wasn't any strong arguments or opposition against using
DPDK for control plane functions that I saw.

In any case this patchset is not introducing any new control plane APIs 
that don't already exist within DPDK today, it only enables the creation 
of a new type of virtual PMDs which are linked to the same base 
infrastructure and which can be used to represent VFs in a control plane 
application as we have implemented in this patch set.

> If we relax this policy, I think the representor solution should be
> a real port, not only "for the purposes of control and monitoring".
> It has been asked several times as replies to this series,
> but it is kindly ignored, saying it will be thought later.
> 

I think we have stated in multiple discussions, especially during the
userspace presentation back in September that this solution supports
data path on the representors PMDs, and we have used the
infrastructure proposed here to do exactly what you are asking. As the
representor infrastructure doesn't preclude the support of a data
path, we have used it as it is presented here to implement a data path 
for exception path packets for a prototype vswitch offload implementation.


> I don't see a general agreement on this series so far.
> 

I think the main issue of contention is that there is a
misunderstanding that this implementation only supports control plane
management and monitoring, but that is not the case and it can be used
for full data path representors, with limited or no control plane
functionality if required, at the end of the day the only limitations
are based on what is implemented by the backend base driver were the 
broker is running for the representor ports.