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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

"Eelco Chaudron" <echaudro@redhat.com> writes:

> On 26 Jun 2019, at 10:38, Jesper Dangaard Brouer wrote:
>
>> On Tue, 25 Jun 2019 03:19:22 +0000
>> "Machulsky, Zorik" <zorik@amazon.com> wrote:
>>
>>> On 6/23/19, 7:21 AM, "Jesper Dangaard Brouer" <brouer@redhat.com> 
>>> wrote:
>>>
>>>     On Sun, 23 Jun 2019 10:06:49 +0300 <sameehj@amazon.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?

Yeah, I was also going to suggest a helper for that. It doesn't
necessarily need to swap the packet context, it could just return a new
pointer?

-Toke