Re: [PATCH v4] netdev attribute to control xdpgeneric skb linearization

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

On Tue, 3 Mar 2020 16:10:14 -0500
Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:

> On Tue, Mar 3, 2020 at 3:50 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > On Tue, 3 Mar 2020 20:46:55 +0100 Daniel Borkmann wrote:  
> > > Thus, when the data/data_end test fails in generic XDP, the user can
> > > call e.g. bpf_xdp_pull_data(xdp, 64) to make sure we pull in as much as
> > > is needed w/o full linearization and once done the data/data_end can be
> > > repeated to proceed. Native XDP will leave xdp->rxq->skb as NULL, but
> > > later we could perhaps reuse the same bpf_xdp_pull_data() helper for
> > > native with skb-less backing. Thoughts?  
> 
> Something akin to pskb_may_pull sounds like a great solution to me.
> 
> Another approach would be a new xdp_action XDP_NEED_LINEARIZED that
> causes the program to be restarted after linearization. But that is both
> more expensive and less elegant.
> 
> Instead of a sysctl or device option, is this an optimization that
> could be taken based on the program? Specifically, would XDP_FLAGS be
> a path to pass a SUPPORT_SG flag along with the program? I'm not
> entirely familiar with the XDP setup code, so this may be a totally
> off. But from a quick read it seems like generic_xdp_install could
> transfer such a flag to struct net_device.
> 
> > I'm curious why we consider a xdpgeneric-only addition. Is attaching
> > a cls_bpf program noticeably slower than xdpgeneric?  
> 
> This just should not be xdp*generic* only, but allow us to use any XDP
> with large MTU sizes and without having to disable GRO.

This is an important point: "should not be xdp*generic* only".

I really want to see this work for XDP-native *first*, and it seems
that with Daniel's idea, it can can also work for XDP-generic.  As Jakub
also hinted, it seems strange that people are trying to implement this
for XDP-generic, as I don't think there is any performance advantage
over cls_bpf.  We really want this to work from XDP-native.


> I'd still like a way to be able to drop or modify packets before GRO,
> or to signal that a type of packet should skip GRO.

That is a use-case, that we should remember to support.

Samih (cc'ed) is working on adding multi-frame support[1] to XDP-native.
Given the huge interest this thread shows, I think I will dedicate
some of my time to help him out on the actual coding.

For my idea to work[1], we first have storage space for the multi-buffer
references, and I propose we use the skb_shared_info area, that is
available anyhow for XDP_PASS that calls build_skb().  Thus, we first
need to standardize across all XDP drivers, how and where this memory
area is referenced/offset.


[1] https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org
[2] https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org#storage-space-for-multi-buffer-referencessegments
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer