Message-ID: <YTckT0k3MV8+Pte/@lore-desk>
Date:   Tue, 7 Sep 2021 10:35:27 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, shayagr@...zon.com,
        dsahern@...nel.org, brouer@...hat.com, echaudro@...hat.com,
        jasowang@...hat.com, alexander.duyck@...il.com, saeed@...nel.org,
        maciej.fijalkowski@...el.com, magnus.karlsson@...el.com,
        tirthendu.sarkar@...el.com, toke@...hat.com
Subject: Re: [PATCH v12 bpf-next 00/18] mvneta: introduce XDP multi-buffer
 support

> 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.
> > 
> > The main idea for the new multi-buffer layout is to reuse the same
> > structure 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 a SKB from an xdp_{buff,frame}.
> > Converting xdp_frame to SKB and deliver it to the network stack is shown
> > in patch 05/18 (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.
> > 
> 
> The series is looking really close to me. Couple small comments/questions
> inline. Also I think we should call out the potential issues in the cover
> letter with regards to backwards compatibility. Something like, 
> 
> "
> A multi-buffer enabled NIC may receive XDP frames with multiple frags.
> If a BPF program does not understand mb layouts its possible to contrive
> a BPF program that incorrectly views data_end as the end of data when
> there is more data in the payload. Note helpers will generally due the
> correct thing, for example perf_output will consume entire payload. But,
> it is still possible some programs could do the wrong thing even if in
> an edge case. Although we expect most BPF programs not to be impacted
> we can't rule out, you've been warned.

ack, I will add it to the cover letter in v13.

Regards,
Lorenzo

> "
> 
> I can't think of an elegant way around this and it does require at least
> some type of opt-in by increasing the MTU limit so I'm OK with it given
> I think it should impact few (no?) real programs.
> 
> > Typical use cases for this series are:
> > - Jumbo-frames
> > - Packet header split (please see Google���s use-case @ NetDevConf 0x14, [0])
> > - TSO/GRO
> > 
> > The two following ebpf helpers (and related selftests) has been introduced:
> > - bpf_xdp_adjust_data:
> >   Move xdp_md->data and xdp_md->data_end pointers in subsequent fragments
> >   according to the offset provided by the ebpf program. This helper can be
> >   used to read/write values in frame payload.
> > - bpf_xdp_get_buff_len:
> >   Return the total frame size (linear + paged parts)
> > 
> > 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].
> > 
> > Changes since v11:
> > - add missing static to bpf_xdp_get_buff_len_proto structure
> > - fix bpf_xdp_adjust_data helper when offset is smaller than linear area length.
> > 
> > Changes since v10:
> > - move xdp->data to the requested payload offset instead of to the beginning of
> >   the fragment in bpf_xdp_adjust_data()
> > 
> > Changes since v9:
> > - introduce bpf_xdp_adjust_data helper and related selftest
> > - add xdp_frags_size and xdp_frags_tsize fields in skb_shared_info
> > - introduce xdp_update_skb_shared_info utility routine in ordere to not reset
> >   frags array in skb_shared_info converting from a xdp_buff/xdp_frame to a skb 
> > - simplify bpf_xdp_copy routine
> > 
> > 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
> > - introduce XDP_FLAGS_FRAGS_PF_MEMALLOC flag
> > 
> > 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 (15):
> >   net: skbuff: add size metadata to skb_shared_info for xdp
> >   xdp: introduce flags field in xdp_buff/xdp_frame
> >   net: mvneta: update mb bit before passing the xdp buffer to eBPF layer
> >   net: mvneta: simplify mvneta_swbm_add_rx_fragment management
> >   net: xdp: add xdp_update_skb_shared_info utility routine
> >   net: marvell: rely on xdp_update_skb_shared_info utility routine
> >   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
> >   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
> >   net: xdp: introduce bpf_xdp_adjust_data helper
> >   bpf: add bpf_xdp_adjust_data selftest
> > 
> >  drivers/net/ethernet/marvell/mvneta.c         | 204 ++++++++++-------
> >  include/linux/skbuff.h                        |   6 +-
> >  include/net/xdp.h                             |  95 +++++++-
> >  include/uapi/linux/bpf.h                      |  39 ++++
> >  kernel/trace/bpf_trace.c                      |   3 +
> >  net/bpf/test_run.c                            | 117 ++++++++--
> >  net/core/filter.c                             | 213 +++++++++++++++++-
> >  net/core/xdp.c                                |  76 ++++++-
> >  tools/include/uapi/linux/bpf.h                |  39 ++++
> >  .../bpf/prog_tests/xdp_adjust_data.c          |  55 +++++
> >  .../bpf/prog_tests/xdp_adjust_tail.c          | 118 ++++++++++
> >  .../selftests/bpf/prog_tests/xdp_bpf2bpf.c    | 151 +++++++++----
> >  .../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 +-
> >  .../bpf/progs/test_xdp_update_frags.c         |  41 ++++
> >  16 files changed, 1036 insertions(+), 165 deletions(-)
> >  create mode 100644 tools/testing/selftests/bpf/prog_tests/xdp_adjust_data.c
> >  create mode 100644 tools/testing/selftests/bpf/progs/test_xdp_update_frags.c
> > 
> > -- 
> > 2.31.1
> > 
> 
> 

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