On Mon, Nov 14, 2022 at 7:04 PM Stanislav Fomichev <sdf@xxxxxxxxxx> wrote: > > Document all current use-cases and assumptions. > > Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx> > --- > 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 >
