netdev - Re: [PATCHv11 bpf-next 2/5] xdp: add a new helper for dev map multicast support

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Date:   Thu, 10 Sep 2020 19:50:14 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        David Ahern <dsahern@...il.com>,
        Hangbin Liu <liuhangbin@...il.com>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Jiri Benc <jbenc@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>, brouer@...hat.com
Subject: Re: [PATCHv11 bpf-next 2/5] xdp: add a new helper for dev map
 multicast support

On Thu, 10 Sep 2020 11:44:50 +0200
Toke Høiland-Jørgensen <toke@...hat.com> wrote:

> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
> 
> > On Wed, Sep 9, 2020 at 8:30 PM David Ahern <dsahern@...il.com> wrote:  
> >> >
> >> > I think the packets modification (edit dst mac, add vlan tag, etc) should be
> >> > done on egress, which rely on David's XDP egress support.  
> >>
> >> agreed. The DEVMAP used for redirect can have programs attached that
> >> update the packet headers - assuming you want to update them.  
> >
> > Then you folks have to submit them as one set.
> > As-is the programmer cannot achieve correct behavior.  
> 
> The ability to attach a program to devmaps is already there. See:
> 
> fbee97feed9b ("bpf: Add support to attach bpf program to a devmap entry")
> 
> But now that you mention it, it does appear that this series is skipping
> the hook that will actually run such a program. Didn't realise that was
> in the caller of bq_enqueue() and not inside bq_enqueue() itself...

In the first revisions of Ahern's patchset (before fully integrated in
devmap), this was the case, but it changed in some of the last
revisions. (This also lost the sort-n-bulk effect in the process, that
optimize I-cache).  In these earlier revisions it operated on
xdp_frame's.  It would have been a lot easier for Hangbin's patch if
the devmap-prog operated on these xdp_frame's.

Maybe we should change the devmap-prog approach, and run this on the
xdp_frame's (in bq_xmit_all() to be precise) .  Hangbin's patchset
clearly shows that we need this "layer" between running the xdp_prog and
the devmap-prog. 

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