[bpf-next,v5,1/1] doc: DEVMAPs and XDP_REDIRECT

Commit Message

Maryam Tahhan Oct. 24, 2022, 12:43 p.m. UTC

From: Maryam Tahhan <[e-mail]>

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 <[e-mail]>
Reviewed-by: Toke Høiland-Jørgensen <[e-mail]>
Reviewed-by: Donald Hunter <[e-mail]>
Reviewed-by: Bagas Sanjaya <[e-mail]>
Acked-by: Toke Høiland-Jørgensen <[e-mail]>
---
 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

Comments

Jesper Dangaard Brouer Oct. 25, 2022, 3:54 p.m. UTC | #1

On 24/10/2022 14.43, [e-mail] wrote:
> From: Maryam Tahhan <[e-mail]>
> 
> 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 <[e-mail]>
> Reviewed-by: Toke Høiland-Jørgensen <[e-mail]>
> Reviewed-by: Donald Hunter <[e-mail]>
> Reviewed-by: Bagas Sanjaya <[e-mail]>
> Acked-by: Toke Høiland-Jørgensen <[e-mail]>
> ---
>   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

Patch

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
+=====
+
+.. 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
+ 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.
+
+.. 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)
+
+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.
+
+.. 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.
+
+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.
+
+.. 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.
+
+More information about redirection can be found :doc:`redirect`
+
+Examples
+========
+
+Kernel BPF
+----------
+
+The following code snippet shows how to declare a ``BPF_MAP_TYPE_DEVMAP``
+called tx_port.
+
+.. code-block:: c
+
+    struct {
+        __uint(type, BPF_MAP_TYPE_DEVMAP);
+        __type(key, __u32);
+        __type(value, __u32);
+        __uint(max_entries, 256);
+    } tx_port SEC(".maps");
+
+The following code snippet shows how to declare a ``BPF_MAP_TYPE_DEVMAP_HASH``
+called forward_map.
+
+.. code-block:: c
+
+    struct {
+        __uint(type, BPF_MAP_TYPE_DEVMAP_HASH);
+        __type(key, __u32);
+        __type(value, struct bpf_devmap_val);
+        __uint(max_entries, 32);
+    } forward_map SEC(".maps");
+
+.. note::
+
+    The value type in the DEVMAP above is a ``struct bpf_devmap_val``
+
+The following code snippet shows a simple xdp_redirect_map program. This program
+would work with a user space program that populates the devmap ``forward_map`` based
+on ingress ifindexes. The BPF program (below) is redirecting packets using the
+ingress ``ifindex`` as the ``key``.
+
+.. code-block:: c
+
+    SEC("xdp")
+    int xdp_redirect_map_func(struct xdp_md *ctx)
+    {
+        int index = ctx->ingress_ifindex;
+
+        return bpf_redirect_map(&forward_map, index, 0);
+    }
+
+The following code snippet shows a BPF program that is broadcasting packets to
+all the interfaces in the ``tx_port`` devmap.
+
+.. code-block:: c
+
+    SEC("xdp")
+    int xdp_redirect_map_func(struct xdp_md *ctx)
+    {
+        return bpf_redirect_map(&tx_port, 0, BPF_F_BROADCAST | BPF_F_EXCLUDE_INGRESS);
+    }
+
+User space
+----------
+
+The following code snippet shows how to update a devmap called ``tx_port``.
+
+.. code-block:: c
+
+    int update_devmap(int ifindex, int redirect_ifindex)
+    {
+        int ret = -1;
+
+        ret = bpf_map_update_elem(bpf_map__fd(tx_port), &ifindex, &redirect_ifindex, 0);
+        if (ret < 0) {
+            fprintf(stderr, "Failed to update devmap_ value: %s\n",
+                strerror(errno));
+        }
+
+        return ret;
+    }
+
+The following code snippet shows how to update a hash_devmap called ``forward_map``.
+
+.. code-block:: c
+
+    int update_devmap(int ifindex, int redirect_ifindex)
+    {
+        struct bpf_devmap_val devmap_val = { .ifindex = redirect_ifindex };
+        int ret = -1;
+
+        ret = bpf_map_update_elem(bpf_map__fd(forward_map), &ifindex, &devmap_val, 0);
+        if (ret < 0) {
+            fprintf(stderr, "Failed to update devmap_ value: %s\n",
+                strerror(errno));
+        }
+        return ret;
+    }
+
+References
+===========
+
+- https://lwn.net/Articles/728146/
+- https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=6f9d451ab1a33728adb72d7ff66a7b374d665176
+- https://elixir.bootlin.com/linux/latest/source/net/core/filter.c#L4106
diff --git a/Documentation/bpf/redirect.rst b/Documentation/bpf/redirect.rst
new file mode 100644
index 000000000000..ff49a0698707
--- /dev/null
+++ b/Documentation/bpf/redirect.rst
@@ -0,0 +1,45 @@ 
+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2022 Red Hat, Inc.
+
+============
+XDP_REDIRECT
+============
+Supported maps
+--------------
+
+XDP_REDIRECT works with the following map types:
+
+- ``BPF_MAP_TYPE_DEVMAP``
+- ``BPF_MAP_TYPE_DEVMAP_HASH``
+- ``BPF_MAP_TYPE_CPUMAP``
+- ``BPF_MAP_TYPE_XSKMAP``
+
+For more information on these maps, please see the specific map documentation.
+
+Process
+-------
+
+XDP_REDIRECT is a three-step process, implemented as follows:
+
+1. The ``bpf_redirect()`` and ``bpf_redirect_map()`` helpers will lookup the
+   target of the redirect (from the supported map types) and store it (along with
+   some other metadata) in a per-CPU ``struct bpf_redirect_info``.
+
+2. When the program returns the ``XDP_REDIRECT`` return code, the driver will
+   call ``xdp_do_redirect()`` which will use the information in ``struct
+   bpf_redirect_info`` to actually enqueue the frame into a map type-specific
+   bulk queue structure.
+
+3. Before exiting its NAPI poll loop, the driver will call ``xdp_do_flush()``,
+   which will flush all the different bulk queues, thus completing the
+   redirect.
+
+.. note::
+    Not all drivers support transmitting frames after a redirect, and for
+    those that do, not all of them support non-linear frames. Non-linear xdp
+    bufs/frames are bufs/frames that contain more than one fragment.
+
+References
+===========
+
+- https://elixir.bootlin.com/linux/latest/source/net/core/filter.c#L4106