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

[Zvi Effron <zeffron@riotgames.com>]

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.

> +
> +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
>