docs/bpf: Document BPF_MAP_TYPE_CPUMAP map

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

Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Maryam Tahhan <mtahhan@redhat.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221107165207.2682075-2-mtahhan@redhat.com

Documentation/bpf/map_cpumap.rst

@@ -0,0 +1,166 @@
+.. 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
+
+.. kernel-doc:: kernel/bpf/cpumap.c
+ :doc: cpu map
+
+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
+=====
+
+Kernel BPF
+----------
+.. 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.
+
+Userspace
+---------
+.. note::
+    CPUMAP entries can only be updated/looked up/deleted from user space and not
+    from an eBPF program. Trying to call these functions from a kernel eBPF
+    program will result in the program failing to load and a verifier warning.
+
+.. c:function::
+    int bpf_map_update_elem(int fd, 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``.
+
+ .. 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;
+    };
+
+ 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.
+
+.. c:function::
+    int bpf_map_lookup_elem(int fd, const void *key, void *value);
+
+ CPU entries can be retrieved using the ``bpf_map_lookup_elem()``
+ helper.
+
+.. c:function::
+    int bpf_map_delete_elem(int fd, 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.
+
+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);
+    }
+
+Userspace
+---------
+
+The following code snippet shows how to dynamically set the max_entries for a
+CPUMAP to the max number of cpus available on the system.
+
+.. code-block:: c
+
+    int set_max_cpu_entries(struct bpf_map *cpu_map)
+    {
+        if (bpf_map__set_max_entries(cpu_map, libbpf_num_possible_cpus()) < 0) {
+            fprintf(stderr, "Failed to set max entries for cpu_map map: %s",
+                strerror(errno));
+            return -1;
+        }
+        return 0;
+    }
+
+References
+===========
+
+- https://developers.redhat.com/blog/2021/05/13/receive-side-scaling-rss-with-ebpf-and-cpumap#redirecting_into_a_cpumap

kernel/bpf/cpumap.c

@@ -4,13 +4,16 @@
  * Copyright (c) 2017 Jesper Dangaard Brouer, Red Hat Inc.
  */
 
-/* The 'cpumap' is primarily used as a backend map for XDP BPF helper
+/**
+ * DOC: cpu map
+ * The 'cpumap' is primarily used as a backend map for XDP BPF helper
  * call bpf_redirect_map() and XDP_REDIRECT action, like 'devmap'.
  *
- * Unlike devmap which redirects XDP frames out another NIC device,
+ * Unlike devmap which redirects XDP frames out to another NIC device,
  * this map type redirects raw XDP frames to another CPU.  The remote
  * CPU will do SKB-allocation and call the normal network stack.
- *
+ */
+/*
  * This is a scalability and isolation mechanism, that allow
  * separating the early driver network XDP layer, from the rest of the
  * netstack, and assigning dedicated CPUs for this stage.  This