193 строки
6.5 KiB
ReStructuredText
193 строки
6.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0-only
|
|
.. Copyright (C) 2022 Red Hat, Inc.
|
|
|
|
===================
|
|
BPF_MAP_TYPE_XSKMAP
|
|
===================
|
|
|
|
.. note::
|
|
- ``BPF_MAP_TYPE_XSKMAP`` was introduced in kernel version 4.18
|
|
|
|
The ``BPF_MAP_TYPE_XSKMAP`` is used as a backend map for XDP BPF helper
|
|
call ``bpf_redirect_map()`` and ``XDP_REDIRECT`` action, like 'devmap' and 'cpumap'.
|
|
This map type redirects raw XDP frames to `AF_XDP`_ sockets (XSKs), a new type of
|
|
address family in the kernel that allows redirection of frames from a driver to
|
|
user space without having to traverse the full network stack. An AF_XDP socket
|
|
binds to a single netdev queue. A mapping of XSKs to queues is shown below:
|
|
|
|
.. code-block:: none
|
|
|
|
+---------------------------------------------------+
|
|
| xsk A | xsk B | xsk C |<---+ User space
|
|
=========================================================|==========
|
|
| Queue 0 | Queue 1 | Queue 2 | | Kernel
|
|
+---------------------------------------------------+ |
|
|
| Netdev eth0 | |
|
|
+---------------------------------------------------+ |
|
|
| +=============+ | |
|
|
| | key | xsk | | |
|
|
| +---------+ +=============+ | |
|
|
| | | | 0 | xsk A | | |
|
|
| | | +-------------+ | |
|
|
| | | | 1 | xsk B | | |
|
|
| | BPF |-- redirect -->+-------------+-------------+
|
|
| | prog | | 2 | xsk C | |
|
|
| | | +-------------+ |
|
|
| | | |
|
|
| | | |
|
|
| +---------+ |
|
|
| |
|
|
+---------------------------------------------------+
|
|
|
|
.. note::
|
|
An AF_XDP socket that is bound to a certain <netdev/queue_id> will *only*
|
|
accept XDP frames from that <netdev/queue_id>. If an XDP program tries to redirect
|
|
from a <netdev/queue_id> other than what the socket is bound to, the frame will
|
|
not be received on the socket.
|
|
|
|
Typically an XSKMAP is created per netdev. This map contains an array of XSK File
|
|
Descriptors (FDs). The number of array elements is typically set or adjusted using
|
|
the ``max_entries`` map parameter. For AF_XDP ``max_entries`` is equal to the number
|
|
of queues supported by the netdev.
|
|
|
|
.. note::
|
|
Both the map key and map value size must be 4 bytes.
|
|
|
|
Usage
|
|
=====
|
|
|
|
Kernel BPF
|
|
----------
|
|
bpf_redirect_map()
|
|
^^^^^^^^^^^^^^^^^^
|
|
.. code-block:: c
|
|
|
|
long bpf_redirect_map(struct bpf_map *map, u32 key, u64 flags)
|
|
|
|
Redirect the packet to the endpoint referenced by ``map`` at index ``key``.
|
|
For ``BPF_MAP_TYPE_XSKMAP`` this map contains references to XSK FDs
|
|
for sockets attached to a netdev's queues.
|
|
|
|
.. note::
|
|
If the map is empty at an index, the packet is dropped. This means that it is
|
|
necessary to have an XDP program loaded with at least one XSK in the
|
|
XSKMAP to be able to get any traffic to user space through the socket.
|
|
|
|
bpf_map_lookup_elem()
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
.. code-block:: c
|
|
|
|
void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
|
|
|
|
XSK entry references of type ``struct xdp_sock *`` can be retrieved using the
|
|
``bpf_map_lookup_elem()`` helper.
|
|
|
|
User space
|
|
----------
|
|
.. note::
|
|
XSK entries can only be updated/deleted from user space and not from
|
|
a BPF program. Trying to call these functions from a kernel BPF program will
|
|
result in the program failing to load and a verifier warning.
|
|
|
|
bpf_map_update_elem()
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
.. code-block:: c
|
|
|
|
int bpf_map_update_elem(int fd, const void *key, const void *value, __u64 flags)
|
|
|
|
XSK entries can be added or updated using the ``bpf_map_update_elem()``
|
|
helper. The ``key`` parameter is equal to the queue_id of the queue the XSK
|
|
is attaching to. And the ``value`` parameter is the FD value of that socket.
|
|
|
|
Under the hood, the XSKMAP update function uses the XSK FD value to retrieve the
|
|
associated ``struct xdp_sock`` instance.
|
|
|
|
The flags argument can be one of the following:
|
|
|
|
- BPF_ANY: Create a new element or update an existing element.
|
|
- BPF_NOEXIST: Create a new element only if it did not exist.
|
|
- BPF_EXIST: Update an existing element.
|
|
|
|
bpf_map_lookup_elem()
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
.. code-block:: c
|
|
|
|
int bpf_map_lookup_elem(int fd, const void *key, void *value)
|
|
|
|
Returns ``struct xdp_sock *`` or negative error in case of failure.
|
|
|
|
bpf_map_delete_elem()
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
.. code-block:: c
|
|
|
|
int bpf_map_delete_elem(int fd, const void *key)
|
|
|
|
XSK entries can be deleted using the ``bpf_map_delete_elem()``
|
|
helper. This helper will return 0 on success, or negative error in case of
|
|
failure.
|
|
|
|
.. note::
|
|
When `libxdp`_ deletes an XSK it also removes the associated socket
|
|
entry from the XSKMAP.
|
|
|
|
Examples
|
|
========
|
|
Kernel
|
|
------
|
|
|
|
The following code snippet shows how to declare a ``BPF_MAP_TYPE_XSKMAP`` called
|
|
``xsks_map`` and how to redirect packets to an XSK.
|
|
|
|
.. code-block:: c
|
|
|
|
struct {
|
|
__uint(type, BPF_MAP_TYPE_XSKMAP);
|
|
__type(key, __u32);
|
|
__type(value, __u32);
|
|
__uint(max_entries, 64);
|
|
} xsks_map SEC(".maps");
|
|
|
|
|
|
SEC("xdp")
|
|
int xsk_redir_prog(struct xdp_md *ctx)
|
|
{
|
|
__u32 index = ctx->rx_queue_index;
|
|
|
|
if (bpf_map_lookup_elem(&xsks_map, &index))
|
|
return bpf_redirect_map(&xsks_map, index, 0);
|
|
return XDP_PASS;
|
|
}
|
|
|
|
User space
|
|
----------
|
|
|
|
The following code snippet shows how to update an XSKMAP with an XSK entry.
|
|
|
|
.. code-block:: c
|
|
|
|
int update_xsks_map(struct bpf_map *xsks_map, int queue_id, int xsk_fd)
|
|
{
|
|
int ret;
|
|
|
|
ret = bpf_map_update_elem(bpf_map__fd(xsks_map), &queue_id, &xsk_fd, 0);
|
|
if (ret < 0)
|
|
fprintf(stderr, "Failed to update xsks_map: %s\n", strerror(errno));
|
|
|
|
return ret;
|
|
}
|
|
|
|
For an example on how create AF_XDP sockets, please see the AF_XDP-example and
|
|
AF_XDP-forwarding programs in the `bpf-examples`_ directory in the `libxdp`_ repository.
|
|
For a detailed explaination of the AF_XDP interface please see:
|
|
|
|
- `libxdp-readme`_.
|
|
- `AF_XDP`_ kernel documentation.
|
|
|
|
.. note::
|
|
The most comprehensive resource for using XSKMAPs and AF_XDP is `libxdp`_.
|
|
|
|
.. _libxdp: https://github.com/xdp-project/xdp-tools/tree/master/lib/libxdp
|
|
.. _AF_XDP: https://www.kernel.org/doc/html/latest/networking/af_xdp.html
|
|
.. _bpf-examples: https://github.com/xdp-project/bpf-examples
|
|
.. _libxdp-readme: https://github.com/xdp-project/xdp-tools/tree/master/lib/libxdp#using-af_xdp-sockets
|