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

[Stanislav Fomichev <sdf@google.com>]

On Tue, Nov 15, 2022 at 3:34 PM Zvi Effron <zeffron@riotgames.com> wrote:
>
> On Tue, Nov 15, 2022 at 2:44 PM Stanislav Fomichev <sdf@google.com> wrote:
> >
> > 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.
> >
>
> That's correct, but it's still a separate program that consumes the metadata,
> unrelated to anything kfuncs. Prior to the existence of kfuncs and AF_XDP, this
> was (to my knowledge) the primary consumer (outside of the original program, of
> course) of the metadata.

SG. I'll add this #4 in the respin and will add a short note that the
tail call operates on the same ctx.

> From the name of the file and commit message, it sounds like this is the
> documentation for XDP metadata, not the documentation for XDP metadata as used
> by kfuncs to implement xdp-hints. Is that correct?

I'm mostly focused on the kfunc-related details for now.



> > > > +
> > > > +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
> > > >