Date:   Tue, 10 Nov 2020 23:53:41 +0000
From:   "Patel, Vedang" <vedang.patel@...el.com>
To:     Saeed Mahameed <saeed@...nel.org>
CC:     Jesper Dangaard Brouer <jbrouer@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Gomes, Vinicius" <vinicius.gomes@...el.com>,
        "Guedes, Andre" <andre.guedes@...el.com>
Subject: Re: Hardware time stamping support for AF_XDP applications

Hi Saeed, 

> On Nov 10, 2020, at 3:32 PM, Saeed Mahameed <saeed@...nel.org> wrote:
> 
> On Tue, 2020-11-10 at 22:44 +0000, Patel, Vedang wrote:
>> [Sorry if you got the email twice. Resending because it was rejected
>> by netdev for containing HTML]
>> 
>> Hi Saeed/Jesper, 
>> 
>> I am working in the Time Sensitive Networking team at Intel. We work
>> on implementing and upstreaming support for TSN related features for
>> intel based NICs. Recently we have been adding support for XDP in
>> i225. One of the features which we want to add support for is passing
>> the hardware timestamp information to the userspace application
>> running AF_XDP sockets (for both Tx and Rx). I came across the XDP
>> Workshop[1] conducted in July 2020 and there you stated that you are
>> already working on adding support for BTF based metadata to pass
>> hardware hints for XDP Programs. My understanding (along with a few
>> questions) of the current state is: 
> 
> Hi Patel,
> 
>> * This feature is currently being maintained out of tree. I found
>> that an RFC Series[2] was posted in June 2018. Are you planning to
>> post an updated version to be merged in the mainline anytime soon? 
> 
> Yes hopefully in the coming couple of weeks.
> 
Sure! I will start testing/developing on top of your branch mentioned below for now.
>> * I am guessing hardware timestamp is one of the metadata fields
>> which will be eventually supported? [3]
> 
> With BTF formatted metadata it is up to the driver to advertise
> whatever it can/want :)
> so yes.
I have a very basic question here. From what I understand about BTF, I can generate a header file (using bpftool?) containing the BTF data format provided by the driver. If so, how can I design an application which can work with multiple NICs drivers without recompilation? I am guessing there is some sort of “master list” of HW hints the drivers will agree upon?
> 
>> * The Metadata support will be extended to pass on the hardware hints
>> to AF_XDP sockets. Are there any rough plans on what metadata will be
>> transferred?
> 
> AF_XDP is not part of my series, but supporting AF_XDP with metadata
> offlaod is up to the driver to implement, should be straight forward
> and identical to XDP.
> 
> what meta data to pass is up to the driver.
Alright, let me take a closer look at your latest code. I will come back will questions if I have any.
> 
> 
>> * The current plan for Tx side only includes passing data from the
>> application to the driver. Are there any plans to support passing
>> information (like HW TX timestamp) from driver to the Application?
>> 
> 
> you mean for AF_XDP ? i actually haven't thought about this, 
> but we could use TX umem packet buffer headroom to pass TX completion
> metadata to AF_XDP app, or extend the completion queue entries to host
> metadata, i am sure that the 1st approach is preferred, but i am not
> planing to support this in my initial series. 
> 
Yeah, I was thinking of using approach 1 as well for this. Let me first work on the Rx side. Then we can scope this one out.
>> Finally, is there any way I can help in expediting the development
>> and upstreaming of this feature? I have been working on studying how
>> XDP works and can work on implementing some part of this feature if
>> you would like.
>> 
> 
> Sure,
> Please feel free to clone and test the following branch if you add
> support to  your driver and implement offloads for AF_XDP that would be
> awesome, and i will append your patches to my series before submission.
> 
> it is always great to send new features with multiple use cases and
> multi vendor support, this will differently expedite submission and
> acceptance
> 
> My Latest work can be found at:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/saeed/linux.git/log/?h=topic/xdp_metadata3
> 
> Please feel free to send me any questions about the code in private or
> public.
Thanks Saeed for all the information! This is really helpful. :)
> 
> Thanks,
> Saeed.
> 
>> Thanks,
>> Vedang Patel
>> Software Engineer
>> Intel Corporation
>> 
>> [1] - https://netdevconf.info/0x14/session.html?workshop-XDP
>> [2] - 
>> https://patchwork.ozlabs.org/project/netdev/cover/20180627024615.17856-1-saeedm@mellanox.com/
>> [3] - 
>> https://xdp-project.net/#outline-container-Important-medium-term-tasks

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