XDP RX Metadata¶
This document describes how an eXpress Data Path (XDP) program can access hardware metadata related to a packet using a set of helper functions, and how it can pass that metadata on to other consumers.
General Design¶
XDP has access to a set of kfuncs to manipulate the metadata in an XDP frame.
Every device driver that wishes to expose additional packet metadata can
implement these kfuncs. 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_kfunc int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *timestamp)¶
Read XDP frame RX timestamp.
Parameters
const struct xdp_md *ctx
XDP context pointer.
u64 *timestamp
Return value pointer.
Return
Returns 0 on success or
-errno
on error.-EOPNOTSUPP
: means device driver does not implement kfunc-ENODATA
: means no RX-timestamp available for this frame
-
__bpf_kfunc int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, u32 *hash, enum xdp_rss_hash_type *rss_type)¶
Read XDP frame RX hash.
Parameters
const struct xdp_md *ctx
XDP context pointer.
u32 *hash
Return value pointer.
enum xdp_rss_hash_type *rss_type
Return value pointer for RSS type.
Description
The RSS hash type (rss_type) specifies what portion of packet headers NIC
hardware used when calculating RSS hash value. The RSS type can be decoded
via enum xdp_rss_hash_type
either matching on individual L3/L4 bits
XDP_RSS_L*
or by combined traditional RSS Hashing Types
XDP_RSS_TYPE_L*
.
Return
Returns 0 on success or
-errno
on error.-EOPNOTSUPP
: means device driver doesn’t implement kfunc-ENODATA
: means no RX-hash available for this frame
An XDP program can use these kfuncs to read the metadata into stack
variables for its own consumption. Or, to pass the metadata on to other
consumers, an XDP program can store it into the metadata area carried
ahead of the packet. Not all packets will necessary have the requested
metadata available in which case the driver returns -ENODATA
.
Not all kfuncs have to be implemented by the device driver; when not
implemented, the default ones that return -EOPNOTSUPP
will be used
to indicate the device driver have not implemented this kfunc.
Within an XDP frame, the metadata layout (accessed via xdp_buff
) is
as follows:
+----------+-----------------+------+
| headroom | custom metadata | data |
+----------+-----------------+------+
^ ^
| |
xdp_buff->data_meta xdp_buff->data
An XDP program can store individual metadata items into this data_meta
area in whichever format it chooses. Later consumers of the metadata
will have to agree on the format by some out of band contract (like for
the AF_XDP use case, see below).
AF_XDP¶
AF_XDP use-case implies that there is a contract between the BPF
program that redirects XDP frames into the AF_XDP
socket (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. The userspace XSK
consumer computes
xsk_umem__get_data() - METADATA_SIZE
to locate that metadata.
Note, xsk_umem__get_data
is defined in libxdp
and
METADATA_SIZE
is an application-specific constant (AF_XDP
receive
descriptor does _not_ explicitly carry the size of the metadata).
Here is the AF_XDP
consumer layout (note missing data_meta
pointer):
+----------+-----------------+------+
| headroom | 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 the skb
out of the xdp_buff
contents. Currently, every driver has custom kernel code to parse
the descriptors and populate skb
metadata when doing this xdp_buff->skb
conversion, and the XDP metadata is not used by the kernel when building
skbs
. However, TC-BPF programs can access the XDP metadata area using
the data_meta
pointer.
In the future, we’d like to support a case where an XDP program
can override some of the metadata used for building skbs
.
bpf_redirect_map¶
bpf_redirect_map
can redirect the frame to a different device.
Some devices (like virtual ethernet links) support running a second XDP
program after the redirect. However, the final consumer doesn’t have
access to the original hardware descriptor and can’t access any of
the original metadata. The same applies to XDP programs installed
into devmaps and cpumaps.
This means that for redirected packets only custom metadata is
currently supported, which has to be prepared by the initial XDP program
before redirect. If the frame is eventually passed to the kernel, the
skb
created from such a frame won’t have any hardware metadata populated
in its skb
. If such a packet is later redirected into an XSK
,
that will also only have access to the custom metadata.
bpf_tail_call¶
Adding programs that access metadata kfuncs to the BPF_MAP_TYPE_PROG_ARRAY
is currently not supported.
Example¶
See tools/testing/selftests/bpf/progs/xdp_metadata.c
and
tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
for an example of
BPF program that handles XDP metadata.