On 24/10/2022 14.43, mtahhan@xxxxxxxxxx wrote:
From: Maryam Tahhan <mtahhan@xxxxxxxxxx>
Add documentation for BPF_MAP_TYPE_DEVMAP and
BPF_MAP_TYPE_DEVMAP_HASH including kernel version
introduced, usage and examples.
Add documentation that describes XDP_REDIRECT.
Signed-off-by: Maryam Tahhan <mtahhan@xxxxxxxxxx>
Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
Reviewed-by: Donald Hunter <donald.hunter@xxxxxxxxx>
Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx>
Acked-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
---
Documentation/bpf/index.rst | 1 +
Documentation/bpf/map_devmap.rst | 203 +++++++++++++++++++++++++++++++
Documentation/bpf/redirect.rst | 45 +++++++
3 files changed, 249 insertions(+)
create mode 100644 Documentation/bpf/map_devmap.rst
create mode 100644 Documentation/bpf/redirect.rst
diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst
index 1b50de1983ee..1088d44634d6 100644
--- a/Documentation/bpf/index.rst
+++ b/Documentation/bpf/index.rst
@@ -29,6 +29,7 @@ that goes into great technical depth about the BPF Architecture.
clang-notes
linux-notes
other
+ redirect
.. only:: subproject and html
diff --git a/Documentation/bpf/map_devmap.rst b/Documentation/bpf/map_devmap.rst
new file mode 100644
index 000000000000..1c6025ea5fdc
--- /dev/null
+++ b/Documentation/bpf/map_devmap.rst
@@ -0,0 +1,203 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2022 Red Hat, Inc.
+
+=================================================
+BPF_MAP_TYPE_DEVMAP and BPF_MAP_TYPE_DEVMAP_HASH
+=================================================
+
+.. note::
+ - ``BPF_MAP_TYPE_DEVMAP`` was introduced in kernel version 4.14
+ - ``BPF_MAP_TYPE_DEVMAP_HASH`` was introduced in kernel version 5.4
+
+``BPF_MAP_TYPE_DEVMAP`` and ``BPF_MAP_TYPE_DEVMAP_HASH`` are BPF maps primarily
+used as backend maps for the XDP BPF helper call ``bpf_redirect_map()``.
+``BPF_MAP_TYPE_DEVMAP`` is backed by an array that uses the key as
+the index to lookup a reference to a net device. While ``BPF_MAP_TYPE_DEVMAP_HASH``
+is backed by a hash table that uses a key to lookup a reference to a net device.
+The user provides either <``key``/ ``ifindex``> or <``key``/ ``struct bpf_devmap_val``>
+pairs to update the maps with new net devices.
+
+.. note::
+ - The key to a hash map doesn't have to be an ``ifindex``.
+ - While ``BPF_MAP_TYPE_DEVMAP_HASH`` allows for densely packing the net devices
+ it comes at the cost of a hash of the key when performing a look up.
+
+The setup and packet enqueue/send code is shared between the two types of
+devmap; only the lookup and insertion is different.
+
+Usage
+=====
Again is this usage section describing BPF-prog kernel-side API or
userspace syscall API ?
+
+.. c:function::
+ long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
+
+ Net device entries can be added or updated using the ``bpf_map_update_elem()``
+ helper. This helper replaces existing elements atomically. The ``value`` parameter
IMHO if we consistently call "helper" for "BPF-helper" the reader will
know this is BPF-prog code. (Feel free to disagree with me on this)
+ can be ``struct bpf_devmap_val`` or a simple ``int ifindex`` for backwards
+ compatibility.
+
+.. note::
+ The maps can only be updated from user space and not from a BPF program.
So, we cannot update this from a BPF-prog, but above API doesn't take a
file-descriptor so it must be the BPF-prog API being describe. That
seems strange, as note says we cannot use this from a BPF-prog...
+.. code-block:: c
+
+ struct bpf_devmap_val {
+ __u32 ifindex; /* device index */
+ union {
+ int fd; /* prog fd on map write */
+ __u32 id; /* prog id on map read */
+ } bpf_prog;
+ };
+
+DEVMAPs can associate a program with a device entry by adding a ``bpf_prog.fd``
+to ``struct bpf_devmap_val``. Programs are run after ``XDP_REDIRECT`` and have
+access to both Rx device and Tx device. The program associated with the ``fd``
+must have type XDP with expected attach type ``xdp_devmap``.
+When a program is associated with a device index, the program is run on an
+``XDP_REDIRECT`` and before the buffer is added to the per-cpu queue. Examples
+of how to attach/use xdp_devmap progs can be found in the kernel selftests:
+
+- ``tools/testing/selftests/bpf/prog_tests/xdp_devmap_attach.c``
+- ``tools/testing/selftests/bpf/progs/test_xdp_with_devmap_helpers.c``
+
+.. c:function::
+ void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
+
This must be BPF-prog code as userspace bpf_map_lookup_elem() have other
parameters. IIRC we did open up for BPF-prog being able to do this
lookup from BPF-prog code, but it doesn't make much sense to do the
lookup (from a performance perspective).
For these map types the bpf_redirect_map() BPF-helper call will (in
recent kernels) do the lookup in the context of the BPF-prog and return
code from bpf_redirect_map() indicate if the element in the map was
available and redirect possible. This API detail should likely also be
documented here (note it is documented in man bpf-helpers(7)).
+net device entries can be retrieved using the ``bpf_map_lookup_elem()``
+helper.
+
+.. c:function::
+ long bpf_map_delete_elem(struct bpf_map *map, const void *key)
+
+net device 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.
Hmm... again this must be BPF-prog code given that API have a map
pointer. I would recommend against changing these maps types from the
packet data-plane code, IMHO this should be left up to the userspace
control-plane.
+
+.. c:function::
+ 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_DEVMAP`` and ``BPF_MAP_TYPE_DEVMAP_HASH`` this map contains
+references to net devices (for forwarding packets through other ports).
+
+The lower two bits of *flags* are used as the return code if the map lookup
+fails. This is so that the return value can be one of the XDP program return
+codes up to ``XDP_TX``, as chosen by the caller. The higher bits of ``flags``
+can be set to ``BPF_F_BROADCAST`` or ``BPF_F_EXCLUDE_INGRESS`` as defined
+below.
Sorry, I see that you already *have* documented the "hidden" lookup in
the bpf_redirect_map() call.
+
+With ``BPF_F_BROADCAST`` the packet will be broadcast to all the interfaces
+in the map, with ``BPF_F_EXCLUDE_INGRESS`` the ingress interface will be excluded
+from the broadcast.
+
Nice that we get documented that XDP can do broadcasting, and via
multiple devmap's also multicast.
+.. note::
+ The key is ignored if BPF_F_BROADCAST is set.
+
+This helper will return ``XDP_REDIRECT`` on success, or the value of the two
+lower bits of the *flags* argument if the map lookup fails.
The "lower two bits" is kind of an implementation detail (which is kind
of a co-incidence as the xdp_action's are not bit fields.)
I made sure that BPF_F_BROADCAST uses value 8, which would allow us to
add more xdp_action's later
$ pahole xdp_action
enum xdp_action {
XDP_ABORTED = 0,
XDP_DROP = 1,
XDP_PASS = 2,
XDP_TX = 3,
XDP_REDIRECT = 4,
};
+More information about redirection can be found :doc:`redirect`
I cannot remember the effect of this :doc:`redirect`, does it link to
redirect.rst file?
--Jesper
