netdev - Re: XDP multi-buffer incl. jumbo-frames (Was: [RFC V1 net-next 1/1] net: ena: implement XDP drop support)

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Date:   Fri, 28 Jun 2019 09:14:39 +0200
From:   "Eelco Chaudron" <echaudro@...hat.com>
To:     "Jesper Dangaard Brouer" <brouer@...hat.com>
Cc:     "Machulsky, Zorik" <zorik@...zon.com>,
        "Jubran, Samih" <sameehj@...zon.com>, davem@...emloft.net,
        netdev@...r.kernel.org, "Woodhouse, David" <dwmw@...zon.co.uk>,
        "Matushevsky, Alexander" <matua@...zon.com>,
        "Bshara, Saeed" <saeedb@...zon.com>,
        "Wilson, Matt" <msw@...zon.com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        "Bshara, Nafea" <nafea@...zon.com>,
        "Tzalik, Guy" <gtzalik@...zon.com>,
        "Belgazal, Netanel" <netanel@...zon.com>,
        "Saidi, Ali" <alisaidi@...zon.com>,
        "Herrenschmidt, Benjamin" <benh@...zon.com>,
        "Kiyanovski, Arthur" <akiyano@...zon.com>,
        "Daniel Borkmann" <borkmann@...earbox.net>,
        "Toke Høiland-Jørgensen" <toke@...hat.com>,
        "Ilias Apalodimas" <ilias.apalodimas@...aro.org>,
        "Alexei Starovoitov" <alexei.starovoitov@...il.com>,
        "Jakub Kicinski" <jakub.kicinski@...ronome.com>,
        xdp-newbies@...r.kernel.org
Subject: Re: XDP multi-buffer incl. jumbo-frames (Was: [RFC V1 net-next 1/1]
 net: ena: implement XDP drop support)



On 26 Jun 2019, at 10:38, Jesper Dangaard Brouer wrote:

> On Tue, 25 Jun 2019 03:19:22 +0000
> "Machulsky, Zorik" <zorik@...zon.com> wrote:
>
>> On 6/23/19, 7:21 AM, "Jesper Dangaard Brouer" <brouer@...hat.com> 
>> wrote:
>>
>>     On Sun, 23 Jun 2019 10:06:49 +0300 <sameehj@...zon.com> wrote:
>>
>>     > This commit implements the basic functionality of drop/pass 
>> logic in the
>>     > ena driver.
>>
>>     Usually we require a driver to implement all the XDP return 
>> codes,
>>     before we accept it.  But as Daniel and I discussed with Zorik 
>> during
>>     NetConf[1], we are going to make an exception and accept the 
>> driver
>>     if you also implement XDP_TX.
>>
>>     As we trust that Zorik/Amazon will follow and implement 
>> XDP_REDIRECT
>>     later, given he/you wants AF_XDP support which requires 
>> XDP_REDIRECT.
>>
>> Jesper, thanks for your comments and very helpful discussion during
>> NetConf! That's the plan, as we agreed. From our side I would like to
>> reiterate again the importance of multi-buffer support by xdp frame.
>> We would really prefer not to see our MTU shrinking because of xdp
>> support.
>
> Okay we really need to make a serious attempt to find a way to support
> multi-buffer packets with XDP. With the important criteria of not
> hurting performance of the single-buffer per packet design.
>
> I've created a design document[2], that I will update based on our
> discussions: [2] 
> https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org
>
> The use-case that really convinced me was Eric's packet header-split.
>
>
> Lets refresh: Why XDP don't have multi-buffer support:
>
> XDP is designed for maximum performance, which is why certain 
> driver-level
> use-cases were not supported, like multi-buffer packets (like 
> jumbo-frames).
> As it e.g. complicated the driver RX-loop and memory model handling.
>
> The single buffer per packet design, is also tied into eBPF 
> Direct-Access
> (DA) to packet data, which can only be allowed if the packet memory is 
> in
> contiguous memory.  This DA feature is essential for XDP performance.
>
>
> One way forward is to define that XDP only get access to the first
> packet buffer, and it cannot see subsequent buffers.  For XDP_TX and
> XDP_REDIRECT to work then XDP still need to carry pointers (plus
> len+offset) to the other buffers, which is 16 bytes per extra buffer.


I’ve seen various network processor HW designs, and they normally get 
the first x bytes (128 - 512) which they can manipulate 
(append/prepend/insert/modify/delete).

There are designs where they can “page in” the additional fragments 
but it’s expensive as it requires additional memory transfers. But the 
majority do not care (cannot change) the remaining fragments. Can also 
not think of a reason why you might want to remove something at the end 
of the frame (thinking about routing/forwarding needs here).

If we do want XDP to access other fragments we could do this through a 
helper which swaps the packet context?

//Eelco