Re: [RFC PATCH v2 00/14] Introducing AF_XDP support

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Thu, 29 Mar 2018 08:16:23 +0200 Björn Töpel <bjorn.topel@gmail.com> wrote:

> 2018-03-28 23:18 GMT+02:00 Eric Leblond <eric@regit.org>:
> > Hello,
> >
> > On Tue, 2018-03-27 at 18:59 +0200, Björn Töpel wrote:  
> >> From: Björn Töpel <bjorn.topel@intel.com>
> >>
> >>  
> > optimized for high performance packet processing and, in upcoming  
> >> patch sets, zero-copy semantics. In this v2 version, we have removed
> >> all zero-copy related code in order to make it smaller, simpler and
> >> hopefully more review friendly. This RFC only supports copy-mode for
> >> the generic XDP path (XDP_SKB) for both RX and TX and copy-mode for
> >> RX
> >>  
> >
> > ...  
> >>
> >> How is then packets distributed between these two XSK? We have
> >> introduced a new BPF map called XSKMAP (or BPF_MAP_TYPE_XSKMAP in
> >> full). The user-space application can place an XSK at an arbitrary
> >> place in this map. The XDP program can then redirect a packet to a
> >> specific index in this map and at this point XDP validates that the
> >> XSK in that map was indeed bound to that device and queue number. If
> >> not, the packet is dropped. If the map is empty at that index, the
> >> packet is also dropped. This also means that it is currently
> >> mandatory
> >> to have an XDP program loaded (and one XSK in the XSKMAP) to be able
> >> to get any traffic to user space through the XSK.  
> >
> > If I get it correctly, this feature will have to be used to bound
> > multiple sockets to a single queue and the eBPF filter will be
> > responsible of the load balancing. Am I correct ?
> >  
> 
> Exactly! The XDP program executing for a certain Rx queue will
> distribute the packets to the socket(s) in the xskmap.

It is important to understand that we (want/need to) maintain a Single
Producer Single Consumer (SPSC) scenario here, for performance reasons.

This _is_ maintained in this patchset AFAIK. But as the API user, you
have to understand that the responsibility of aligning this is yours!
If you don't the frames are dropped (silently).
  The BPF programmer MUST select the correct XSKMAP index,  such that
the ctx->rx_queue_index match the queue_id registered in the xdp_sock
(and bounded ifindex also match).


Bjørn, Magnus and I have discussed other API options. E.g. where the
XSKMAP index _is_ the rx_queue_index, and BPF programmer is not allowed
select another index.  We settled on the API in the patchset, where BPF
programmer get more freedom, and can select an invalid index, that
cause packets to be dropped.

An advantage of this API is that we allow one RX-queue, to multiplex
into many xdk_sock's (all bound to this same RX-queue and ifindex).
This still maintain a Single Producer, as the RX-queue just have a
Single Producer relationship's with each xdp_sock.


I imagine, that Suricata/Eric, want to capture all the RX-queues on
the net_device.  For this to happen, he need to create a xdp_sock per
RX-queue, and have a side-bpf-map that assist in the XSKMAP lookup, or
simply populate the XSKMAP to correspond to the rx_queue_index.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer