Re: [PATCH bpf-next 01/11] bpf: Document XDP RX metadata

[Stanislav Fomichev <sdf@google.com>]

On Tue, Nov 15, 2022 at 2:31 PM Zvi Effron <zeffron@riotgames.com> wrote:
>
> On Mon, Nov 14, 2022 at 7:04 PM Stanislav Fomichev <sdf@google.com> wrote:
> >
> > Document all current use-cases and assumptions.
> >
> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > ---
> >  Documentation/bpf/xdp-rx-metadata.rst | 109 ++++++++++++++++++++++++++
> >  1 file changed, 109 insertions(+)
> >  create mode 100644 Documentation/bpf/xdp-rx-metadata.rst
> >
> > diff --git a/Documentation/bpf/xdp-rx-metadata.rst b/Documentation/bpf/xdp-rx-metadata.rst
> > new file mode 100644
> > index 000000000000..5ddaaab8de31
> > --- /dev/null
> > +++ b/Documentation/bpf/xdp-rx-metadata.rst
> > @@ -0,0 +1,109 @@
> > +===============
> > +XDP RX Metadata
> > +===============
> > +
> > +XDP programs support creating and passing custom metadata via
> > +``bpf_xdp_adjust_meta``. This metadata can be consumed by the following
> > +entities:
> > +
> > +1. ``AF_XDP`` consumer.
> > +2. Kernel core stack via ``XDP_PASS``.
> > +3. Another device via ``bpf_redirect_map``.
>
> 4. Other eBPF programs via eBPF tail calls.

Don't think a tail call is a special case here?
Correct me if I'm wrong, but with a tail call, we retain the original
xdp_buff ctx, so the tail call can still use the same kfuncs as if the
original bpf prog was running.

> > +
> > +General Design
> > +==============
> > +
> > +XDP has access to a set of kfuncs to manipulate the metadata. Every
> > +device driver implements these kfuncs by generating BPF bytecode
> > +to parse it out from the hardware descriptors. The set of kfuncs is
> > +declared in ``include/net/xdp.h`` via ``XDP_METADATA_KFUNC_xxx``.
> > +
> > +Currently, the following kfuncs are supported. In the future, as more
> > +metadata is supported, this set will grow:
> > +
> > +- ``bpf_xdp_metadata_rx_timestamp_supported`` returns true/false to
> > +  indicate whether the device supports RX timestamps in general
> > +- ``bpf_xdp_metadata_rx_timestamp`` returns packet RX timestamp or 0
> > +- ``bpf_xdp_metadata_export_to_skb`` prepares metadata layout that
> > +  the kernel will be able to consume. See ``bpf_redirect_map`` section
> > +  below for more details.
> > +
> > +Within the XDP frame, the metadata layout is as follows::
> > +
> > +  +----------+------------------+-----------------+------+
> > +  | headroom | xdp_skb_metadata | custom metadata | data |
> > +  +----------+------------------+-----------------+------+
> > +                                ^                 ^
> > +                                |                 |
> > +                      xdp_buff->data_meta   xdp_buff->data
> > +
> > +Where ``xdp_skb_metadata`` is the metadata prepared by
> > +``bpf_xdp_metadata_export_to_skb``. And ``custom metadata``
> > +is prepared by the BPF program via calls to ``bpf_xdp_adjust_meta``.
> > +
> > +Note that ``bpf_xdp_metadata_export_to_skb`` doesn't adjust
> > +``xdp->data_meta`` pointer. To access the metadata generated
> > +by ``bpf_xdp_metadata_export_to_skb`` use ``xdp_buf->skb_metadata``.
> > +
> > +AF_XDP
> > +======
> > +
> > +``AF_XDP`` use-case implies that there is a contract between the BPF program
> > +that redirects XDP frames into the ``XSK`` and the final consumer.
> > +Thus the BPF program manually allocates a fixed number of
> > +bytes out of metadata via ``bpf_xdp_adjust_meta`` and calls a subset
> > +of kfuncs to populate it. User-space ``XSK`` consumer, looks
> > +at ``xsk_umem__get_data() - METADATA_SIZE`` to locate its metadata.
> > +
> > +Here is the ``AF_XDP`` consumer layout (note missing ``data_meta`` pointer)::
> > +
> > +  +----------+------------------+-----------------+------+
> > +  | headroom | xdp_skb_metadata | custom metadata | data |
> > +  +----------+------------------+-----------------+------+
> > +                                                  ^
> > +                                                  |
> > +                                           rx_desc->address
> > +
> > +XDP_PASS
> > +========
> > +
> > +This is the path where the packets processed by the XDP program are passed
> > +into the kernel. The kernel creates ``skb`` out of the ``xdp_buff`` contents.
> > +Currently, every driver has a custom kernel code to parse the descriptors and
> > +populate ``skb`` metadata when doing this ``xdp_buff->skb`` conversion.
> > +In the future, we'd like to support a case where XDP program can override
> > +some of that metadata.
> > +
> > +The plan of record is to make this path similar to ``bpf_redirect_map``
> > +below where the program would call ``bpf_xdp_metadata_export_to_skb``,
> > +override the metadata and return ``XDP_PASS``. Additional work in
> > +the drivers will be required to enable this (for example, to skip
> > +populating ``skb`` metadata from the descriptors when
> > +``bpf_xdp_metadata_export_to_skb`` has been called).
> > +
> > +bpf_redirect_map
> > +================
> > +
> > +``bpf_redirect_map`` can redirect the frame to a different device.
> > +In this case we don't know ahead of time whether that final consumer
> > +will further redirect to an ``XSK`` or pass it to the kernel via ``XDP_PASS``.
> > +Additionally, the final consumer doesn't have access to the original
> > +hardware descriptor and can't access any of the original metadata.
> > +
> > +To support passing metadata via ``bpf_redirect_map``, there is a
> > +``bpf_xdp_metadata_export_to_skb`` kfunc that populates a subset
> > +of metadata into ``xdp_buff``. The layout is defined in
> > +``struct xdp_skb_metadata``.
> > +
> > +Mixing custom metadata and xdp_skb_metadata
> > +===========================================
> > +
> > +For the cases of ``bpf_redirect_map``, where the final consumer isn't
> > +known ahead of time, the program can store both, custom metadata
> > +and ``xdp_skb_metadata`` for the kernel consumption.
> > +
> > +Current limitation is that the program cannot adjust ``data_meta`` (via
> > +``bpf_xdp_adjust_meta``) after a call to ``bpf_xdp_metadata_export_to_skb``.
> > +So it has to, first, prepare its custom metadata layout and only then,
> > +optionally, store ``xdp_skb_metadata`` via a call to
> > +``bpf_xdp_metadata_export_to_skb``.
> > --
> > 2.38.1.431.g37b22c650d-goog
> >