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>]

Jesper Dangaard Brouer <brouer@redhat.com> writes:

> 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.

Yeah, I think this would be reasonable. As long as we can have a
metadata field with the full length + still give XDP programs the
ability to truncate the packet (i.e., discard the subsequent pages) I
think many (most?) use cases will work fine without having access to the
full packet data...

-Toke