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 Wed, 26 Jun 2019 13:52:16 +0200
> Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
>> 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)
>
> You touch upon some interesting complications already:
>
> 1. It is valuable for XDP bpf_prog to know "full" length?
>    (if so, then we need to extend xdp ctx with info)

Valuable, quite likely. A hard requirement, probably not (for all use
cases).

>  But if we need to know the full length, when the first-buffer is
>  processed. Then realize that this affect the drivers RX-loop, because
>  then we need to "collect" all the buffers before we can know the
>  length (although some HW provide this in first descriptor).
>
>  We likely have to change drivers RX-loop anyhow, as XDP_TX and
>  XDP_REDIRECT will also need to "collect" all buffers before the packet
>  can be forwarded. (Although this could potentially happen later in
>  driver loop when it meet/find the End-Of-Packet descriptor bit).
>  
>
> 2. Can we even allow helper bpf_xdp_adjust_tail() ?
>
>  Wouldn't it be easier to disallow a BPF-prog with this helper, when
>  driver have configured multi-buffer?

Easier, certainly. But then it's even easier to not implement this at
all ;)

>  Or will it be too restrictive, if jumbo-frame is very uncommon and
>  only enabled because switch infra could not be changed (like Amazon
>  case).

I think it would be preferable to support it; but maybe we can let that
depend on how difficult it actually turns out to be to allow it?

>  Perhaps it is better to let bpf_xdp_adjust_tail() fail runtime?

If we do disallow it, I think I'd lean towards failing the call at
runtime...

-Toke