On Tue, Nov 15, 2022 at 2:31 PM Zvi Effron <zeffron@xxxxxxxxxxxxx> wrote: > > 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. 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 > >
