Date:   Thu, 1 Jul 2021 09:56:40 +0200
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, shayagr@...zon.com,
        sameehj@...zon.com, David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Saeed Mahameed <saeed@...nel.org>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Tirthendu <tirthendu.sarkar@...el.com>
Subject: Re: [PATCH v9 bpf-next 00/14] mvneta: introduce XDP multi-buffer support

On Wed, Jun 23, 2021 at 1:19 AM John Fastabend <john.fastabend@...il.com> wrote:
>
> Lorenzo Bianconi wrote:
> > This series introduce XDP multi-buffer support. The mvneta driver is
> > the first to support these new "non-linear" xdp_{buff,frame}. Reviewers
> > please focus on how these new types of xdp_{buff,frame} packets
> > traverse the different layers and the layout design. It is on purpose
> > that BPF-helpers are kept simple, as we don't want to expose the
> > internal layout to allow later changes.
> >
> > For now, to keep the design simple and to maintain performance, the XDP
> > BPF-prog (still) only have access to the first-buffer. It is left for
> > later (another patchset) to add payload access across multiple buffers.
> > This patchset should still allow for these future extensions. The goal
> > is to lift the XDP MTU restriction that comes with XDP, but maintain
> > same performance as before.
>
> At this point I don't think we can have a partial implementation. At
> the moment we have packet capture applications and protocol parsers
> running in production. If we allow this to go in staged we are going
> to break those applications that make the fundamental assumption they
> have access to all the data in the packet.
>
> There will be no way to fix it when it happens. The teams running the
> applications wont necessarily be able to change the network MTU. Now
> it doesn't work, hard stop. This is better than it sort of works some
> of the time. Worse if we get in a situation where some drivers support
> partial access and others support full access the support matrix gets worse.
>
> I think we need to get full support and access to all bytes. I believe
> I said this earlier, but now we've deployed apps that really do need
> access to the payloads so its not a theoritical concern anymore, but
> rather a real one based on deployed BPF programs.
>
> >
> > The main idea for the new multi-buffer layout is to reuse the same
> > layout used for non-linear SKB. This rely on the "skb_shared_info"
> > struct at the end of the first buffer to link together subsequent
> > buffers. Keeping the layout compatible with SKBs is also done to ease
> > and speedup creating an SKB from an xdp_{buff,frame}.
> > Converting xdp_frame to SKB and deliver it to the network stack is shown
> > in patch 07/14 (e.g. cpumaps).
> >
> > A multi-buffer bit (mb) has been introduced in the flags field of xdp_{buff,frame}
> > structure to notify the bpf/network layer if this is a xdp multi-buffer frame
> > (mb = 1) or not (mb = 0).
> > The mb bit will be set by a xdp multi-buffer capable driver only for
> > non-linear frames maintaining the capability to receive linear frames
> > without any extra cost since the skb_shared_info structure at the end
> > of the first buffer will be initialized only if mb is set.
> > Moreover the flags field in xdp_{buff,frame} will be reused even for
> > xdp rx csum offloading in future series.
> >
> > Typical use cases for this series are:
> > - Jumbo-frames
> > - Packet header split (please see Google���s use-case @ NetDevConf 0x14, [0])
> > - TSO
> >
> > A new bpf helper (bpf_xdp_get_buff_len) has been introduce in order to notify
> > the eBPF layer about the total frame size (linear + paged parts).
>
> Is it possible to make currently working programs continue to work?
> For a simple packet capture example a program might capture the
> entire packet of bytes '(data_end - data_start)'. With above implementation
> the program will continue to run, but will no longer be capturing
> all the bytes... so its a silent failure. Otherwise I'll need to
> backport fixes into my BPF programs and releases to ensure they
> don't walk onto a new kernel with multi-buffer support enabled.
> Its not ideal.
>
> >
> > bpf_xdp_adjust_tail and bpf_xdp_copy helpers have been modified to take into
> > account xdp multi-buff frames.
> >
> > More info about the main idea behind this approach can be found here [1][2].
>
> Will read [1],[2].
>
> Where did the perf data for the 40gbps NIC go? I think we want that
> done again on this series with at least 40gbps NICs and better
> yet 100gbps drivers. If its addressed in a patch commit message
> I'm reading the series now.

Here is the perf data for a 40 gbps i40e on my 2.1 GHz Cascade Lake server.

                                xdpsock -r           XDP_DROP        XDP_TX
Lorenzo's patches:  -2%/+1.5 cycles    -3%/+3            +2%/-6 (Yes,
it gets better!)
+ i40e support:        -5.5%/+5               -8%/+9             -9%/+31

It seems that it is the driver support itself that hurts now. The
overhead of the base support has decreased substantially over time
which is good.

> >
> > Changes since v8:
> > - add proper dma unmapping if XDP_TX fails on mvneta for a xdp multi-buff
> > - switch back to skb_shared_info implementation from previous xdp_shared_info
> >   one
> > - avoid using a bietfield in xdp_buff/xdp_frame since it introduces performance
> >   regressions. Tested now on 10G NIC (ixgbe) to verify there are no performance
> >   penalties for regular codebase
> > - add bpf_xdp_get_buff_len helper and remove frame_length field in xdp ctx
> > - add data_len field in skb_shared_info struct
> >
> > Changes since v7:
> > - rebase on top of bpf-next
> > - fix sparse warnings
> > - improve comments for frame_length in include/net/xdp.h
> >
> > Changes since v6:
> > - the main difference respect to previous versions is the new approach proposed
> >   by Eelco to pass full length of the packet to eBPF layer in XDP context
> > - reintroduce multi-buff support to eBPF kself-tests
> > - reintroduce multi-buff support to bpf_xdp_adjust_tail helper
> > - introduce multi-buffer support to bpf_xdp_copy helper
> > - rebase on top of bpf-next
> >
> > Changes since v5:
> > - rebase on top of bpf-next
> > - initialize mb bit in xdp_init_buff() and drop per-driver initialization
> > - drop xdp->mb initialization in xdp_convert_zc_to_xdp_frame()
> > - postpone introduction of frame_length field in XDP ctx to another series
> > - minor changes
> >
> > Changes since v4:
> > - rebase ontop of bpf-next
> > - introduce xdp_shared_info to build xdp multi-buff instead of using the
> >   skb_shared_info struct
> > - introduce frame_length in xdp ctx
> > - drop previous bpf helpers
> > - fix bpf_xdp_adjust_tail for xdp multi-buff
> > - introduce xdp multi-buff self-tests for bpf_xdp_adjust_tail
> > - fix xdp_return_frame_bulk for xdp multi-buff
> >
> > Changes since v3:
> > - rebase ontop of bpf-next
> > - add patch 10/13 to copy back paged data from a xdp multi-buff frame to
> >   userspace buffer for xdp multi-buff selftests
> >
> > Changes since v2:
> > - add throughput measurements
> > - drop bpf_xdp_adjust_mb_header bpf helper
> > - introduce selftest for xdp multibuffer
> > - addressed comments on bpf_xdp_get_frags_count
> > - introduce xdp multi-buff support to cpumaps
> >
> > Changes since v1:
> > - Fix use-after-free in xdp_return_{buff/frame}
> > - Introduce bpf helpers
> > - Introduce xdp_mb sample program
> > - access skb_shared_info->nr_frags only on the last fragment
> >
> > Changes since RFC:
> > - squash multi-buffer bit initialization in a single patch
> > - add mvneta non-linear XDP buff support for tx side
> >
> > [0] https://netdevconf.info/0x14/session.html?talk-the-path-to-tcp-4k-mtu-and-rx-zerocopy
> > [1] https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org
> > [2] https://netdevconf.info/0x14/session.html?tutorial-add-XDP-support-to-a-NIC-driver (XDPmulti-buffers section)
> >
> > Eelco Chaudron (3):
> >   bpf: add multi-buff support to the bpf_xdp_adjust_tail() API
> >   bpf: add multi-buffer support to xdp copy helpers
> >   bpf: update xdp_adjust_tail selftest to include multi-buffer
> >
> > Lorenzo Bianconi (11):
> >   net: skbuff: add data_len field to skb_shared_info
> >   xdp: introduce flags field in xdp_buff/xdp_frame
> >   net: mvneta: update mb bit before passing the xdp buffer to eBPF layer
> >   xdp: add multi-buff support to xdp_return_{buff/frame}
> >   net: mvneta: add multi buffer support to XDP_TX
> >   net: mvneta: enable jumbo frames for XDP
> >   net: xdp: add multi-buff support to xdp_build_skb_from_frame
> >   bpf: introduce bpf_xdp_get_buff_len helper
> >   bpf: move user_size out of bpf_test_init
> >   bpf: introduce multibuff support to bpf_prog_test_run_xdp()
> >   bpf: test_run: add xdp_shared_info pointer in bpf_test_finish
> >     signature
> >
> >  drivers/net/ethernet/marvell/mvneta.c         | 143 ++++++++++------
> >  include/linux/skbuff.h                        |   5 +-
> >  include/net/xdp.h                             |  56 ++++++-
> >  include/uapi/linux/bpf.h                      |   7 +
> >  kernel/trace/bpf_trace.c                      |   3 +
> >  net/bpf/test_run.c                            | 108 +++++++++---
> >  net/core/filter.c                             | 157 +++++++++++++++++-
> >  net/core/xdp.c                                |  72 +++++++-
> >  tools/include/uapi/linux/bpf.h                |   7 +
> >  .../bpf/prog_tests/xdp_adjust_tail.c          | 105 ++++++++++++
> >  .../selftests/bpf/prog_tests/xdp_bpf2bpf.c    | 127 +++++++++-----
> >  .../bpf/progs/test_xdp_adjust_tail_grow.c     |  10 +-
> >  .../bpf/progs/test_xdp_adjust_tail_shrink.c   |  32 +++-
> >  .../selftests/bpf/progs/test_xdp_bpf2bpf.c    |   2 +-
> >  14 files changed, 705 insertions(+), 129 deletions(-)
> >
> > --
> > 2.31.1
> >
>
>

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