[PATCH bpf-next v2 1/1] docs: BPF_MAP_TYPE_CPUMAP

[mtahhan@redhat.com]

From: Maryam Tahhan <mtahhan@redhat.com>

Add documentation for BPF_MAP_TYPE_CPUMAP including
kernel version introduced, usage and examples.

Signed-off-by: Maryam Tahhan <mtahhan@redhat.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
---
 Documentation/bpf/map_cpumap.rst | 140 +++++++++++++++++++++++++++++++
 1 file changed, 140 insertions(+)
 create mode 100644 Documentation/bpf/map_cpumap.rst

diff --git a/Documentation/bpf/map_cpumap.rst b/Documentation/bpf/map_cpumap.rst
new file mode 100644
index 000000000000..23320fb61bf7
--- /dev/null
+++ b/Documentation/bpf/map_cpumap.rst
@@ -0,0 +1,140 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2022 Red Hat, Inc.
+
+===================
+BPF_MAP_TYPE_CPUMAP
+===================
+
+.. note::
+   - ``BPF_MAP_TYPE_CPUMAP`` was introduced in kernel version 4.15
+
+``BPF_MAP_TYPE_CPUMAP`` is primarily used as a backend map for the XDP BPF
+helpers ``bpf_redirect_map()`` and ``XDP_REDIRECT`` action. This map type redirects raw
+XDP frames to another CPU.
+
+A CPUMAP is a scalability and isolation mechanism that allows the steering of packets
+to dedicated CPUs for processing. An example use-case for this map type is software
+based Receive Side Scaling (RSS).
+
+The CPUMAP represents the CPUs in the system indexed as the map-key, and the
+map-value is the config setting (per CPUMAP entry). Each CPUMAP entry has a dedicated
+kernel thread bound to the given CPU to represent the remote CPU execution unit.
+
+Starting from Linux kernel version 5.9 the CPUMAP can run a second XDP program
+on the remote CPU. This allows an XDP program to split its processing across
+multiple CPUs. For example, a scenario where the initial CPU (that sees/receives
+the packets) needs to do minimal packet processing and the remote CPU (to which
+the packet is directed) can afford to spend more cycles processing the frame. The
+initial CPU is where the XDP redirect program is executed. The remote CPU
+receives raw``xdp_frame`` objects.
+
+Usage
+=====
+
+.. c:function::
+   long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
+
+ CPU 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_cpumap_val``.
+
+ .. note::
+    The maps can only be updated from user space and not from a BPF program.
+
+ .. code-block:: c
+
+    struct bpf_cpumap_val {
+        __u32 qsize;  /* queue size to remote target CPU */
+        union {
+            int   fd; /* prog fd on map write */
+            __u32 id; /* prog id on map read */
+        } bpf_prog;
+    };
+
+.. c:function::
+   void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
+
+ CPU 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)
+
+ CPU 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_CPUMAP`` this map contains references to CPUs.
+
+ 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.
+
+Examples
+========
+Kernel
+------
+
+The following code snippet shows how to declare a BPF_MAP_TYPE_CPUMAP called
+cpu_map and how to redirect packets to a remote CPU using a round robin scheme.
+
+.. code-block:: c
+
+   struct {
+        __uint(type, BPF_MAP_TYPE_CPUMAP);
+        __type(key, u32);
+        __type(value, struct bpf_cpumap_val);
+        __uint(max_entries, 12);
+    } cpu_map SEC(".maps");
+
+    struct {
+        __uint(type, BPF_MAP_TYPE_ARRAY);
+        __type(key, u32);
+        __type(value, u32);
+        __uint(max_entries, 12);
+    } cpus_available SEC(".maps");
+
+    struct {
+        __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
+        __type(key, u32);
+        __type(value, u32);
+        __uint(max_entries, 1);
+       } cpus_iterator SEC(".maps");
+
+    SEC("xdp")
+    int  xdp_redir_cpu_round_robin(struct xdp_md *ctx)
+    {
+        u32 key = 0;
+        u32 cpu_dest = 0;
+        u32 *cpu_selected, *cpu_iterator;
+        u32 cpu_idx;
+
+        cpu_iterator = bpf_map_lookup_elem(&cpus_iterator, &key);
+        if (!cpu_iterator)
+            return XDP_ABORTED;
+        cpu_idx = *cpu_iterator;
+
+        *cpu_iterator += 1;
+        if (*cpu_iterator == bpf_num_possible_cpus())
+            *cpu_iterator = 0;
+
+        cpu_selected = bpf_map_lookup_elem(&cpus_available, &cpu_idx);
+        if (!cpu_selected)
+            return XDP_ABORTED;
+        cpu_dest = *cpu_selected;
+
+        if (cpu_dest >= bpf_num_possible_cpus())
+            return XDP_ABORTED;
+
+        return bpf_redirect_map(&cpu_map, cpu_dest, 0);
+    }
+
+References
+===========
+
+- https://elixir.bootlin.com/linux/v6.0.1/source/kernel/bpf/cpumap.c
+- https://developers.redhat.com/blog/2021/05/13/receive-side-scaling-rss-with-ebpf-and-cpumap#redirecting_into_a_cpumap
-- 
2.35.3