[PATCH v2] bpf, docs: document BPF_MAP_TYPE_HASH and variants

[Donald Hunter <donald.hunter@gmail.com>]

Add documentation for BPF_MAP_TYPE_HASH including kernel version
introduced, usage and examples. Document BPF_MAP_TYPE_PERCPU_HASH,
BPF_MAP_TYPE_LRU_HASH and BPF_MAP_TYPE_LRU_PERCPU_HASH variations.

Note that this file is included in the BPF documentation by the glob in
Documentation/bpf/maps.rst

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
---
 Documentation/bpf/map_hash.rst | 181 +++++++++++++++++++++++++++++++++
 1 file changed, 181 insertions(+)
 create mode 100644 Documentation/bpf/map_hash.rst

diff --git a/Documentation/bpf/map_hash.rst b/Documentation/bpf/map_hash.rst
new file mode 100644
index 000000000000..d9e33152dae5
--- /dev/null
+++ b/Documentation/bpf/map_hash.rst
@@ -0,0 +1,181 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2022 Red Hat, Inc.
+
+===============================================
+BPF_MAP_TYPE_HASH, with PERCPU and LRU Variants
+===============================================
+
+.. note::
+   - ``BPF_MAP_TYPE_HASH`` was introduced in kernel version 3.19
+   - ``BPF_MAP_TYPE_PERCPU_HASH`` was introduced in version 4.6
+   - Both ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
+     were introduced in version 4.10
+
+``BPF_MAP_TYPE_HASH`` and ``BPF_MAP_TYPE_PERCPU_HASH`` provide general
+purpose hash map storage. Both the key and the value can be structs,
+allowing for composite keys and values.
+
+The kernel is responsible for allocating and freeing key/value pairs, up
+to the max_entries limit that you specify. Hash maps use pre-allocation
+of hash table elements by default. The ``BPF_F_NO_PREALLOC`` flag can be
+used to disable pre-allocation when it is to memory expensive.
+
+``BPF_MAP_TYPE_PERCPU_HASH`` provides a separate value slot per
+CPU. The per-cpu values are stored internally in an array.
+
+The ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
+variants add LRU semantics to their respective hash tables. An LRU hash
+will automatically evict the least recently used entries when the hash
+table reaches capacity. An LRU hash maintains an internal LRU list that
+is used to select elements for eviction. This internal LRU list is
+shared across CPUs but it is possible to request a per CPU LRU list with
+the ``BPF_F_NO_COMMON_LRU`` flag when calling ``bpf_map_create``.
+
+Usage
+=====
+
+.. c:function::
+   long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
+
+Hash entries can be added or updated using the ``bpf_map_update_elem()``
+helper. This helper replaces existing elements atomically. The ``flags``
+parameter can be used to control the update behaviour:
+
+- ``BPF_ANY`` will create a new element or update an existing element
+- ``BPF_NOTEXIST`` will create a new element only if one did not already
+  exist
+- ``BPF_EXIST`` will update an existing element
+
+``bpf_map_update_elem()`` returns 0 on success, or negative error in
+case of failure.
+
+.. c:function::
+   void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
+
+Hash entries can be retrieved using the ``bpf_map_lookup_elem()``
+helper. This helper returns a pointer to the value associated with
+``key``, or ``NULL`` if no entry was found.
+
+.. c:function::
+   long bpf_map_delete_elem(struct bpf_map *map, const void *key)
+
+Hash 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.
+
+Per CPU Hashes
+--------------
+
+For ``BPF_MAP_TYPE_PERCPU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
+the ``bpf_map_update_elem()`` and ``bpf_map_lookup_elem()`` helpers
+automatically access the hash slot for the current CPU.
+
+.. c:function::
+   void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void *key, u32 cpu)
+
+The ``bpf_map_lookup_percpu_elem()`` helper can be used to lookup the
+value in the hash slot for a specific CPU. Returns value associated with
+``key`` on ``cpu`` , or ``NULL`` if no entry was found or ``cpu`` is
+invalid.
+
+Concurrency
+-----------
+
+Values stored in ``BPF_MAP_TYPE_HASH`` can be accessed concurrently by
+programs running on different CPUs.  Since Kernel version 5.1, the BPF
+infrastructure provides ``struct bpf_spin_lock`` to synchronize access.
+See ``tools/testing/selftests/bpf/progs/test_spin_lock.c``.
+
+Userspace
+---------
+
+.. c:function::
+   int bpf_map_get_next_key (int fd, const void *cur_key, void *next_key)
+
+In userspace, is possible to iterate through the keys of a hash using
+the ``bpf_map_get_next_key()`` function. The first key can be fetched by
+calling ``bpf_map_get_next_key()`` with ``cur_key`` set to
+``NULL``. Subsequent calls will fetch the next key that follows the
+current key. ``bpf_map_get_next_key()`` returns 0 on success, -ENOENT if
+cur_key is the last key in the hash, or negative error in case of
+failure.
+
+Examples
+========
+
+Please see the ``tools/testing/selftests/bpf`` directory for functional
+examples.  The sample code below demonstrates API usage.
+
+This example shows how to declare an LRU Hash with a struct key and a
+struct value.
+
+.. code-block:: c
+
+    #include <linux/bpf.h>
+    #include <bpf/bpf_helpers.h>
+
+    struct key {
+        __u32 srcip;
+    };
+
+    struct value {
+        __u64 packets;
+        __u64 bytes;
+    };
+
+    struct {
+            __uint(type, BPF_MAP_TYPE_LRU_HASH);
+            __uint(max_entries, 32);
+            __type(key, struct key);
+            __type(value, struct value);
+    } packet_stats SEC(".maps");
+
+This example shows how to create or update hash values using atomic
+instructions:
+
+.. code-block:: c
+
+    static inline void (__u32 srcip, int bytes)
+    {
+            struct key key = {
+                    .srcip = srcip
+            };
+            struct value *value = bpf_map_lookup_elem(&packet_stats, &key);
+            if (value) {
+                    __sync_fetch_and_add(&value->packets, 1);
+                    __sync_fetch_and_add(&value->bytes, bytes);
+            } else {
+                    struct value newval = { 1, bytes };
+                    bpf_map_update_elem(&packet_stats, &key, &newval, BPF_NOEXIST);
+            }
+    }
+
+Userspace walking the map elements from the map declared above:
+
+.. code-block:: c
+
+    #include <bpf/libbpf.h>
+    #include <bpf/bpf.h>
+
+    static void walk_hash_elements(int map_fd)
+    {
+            struct key *cur_key = NULL;
+            struct key next_key;
+            int next;
+            do {
+                    // error checking omitted
+                    next = bpf_map_get_next_key(stats_fd, cur_key, &next_key);
+                    if (next == -ENOENT)
+                            break;
+
+                    struct in_addr src_addr = {
+                            .s_addr = next_key.srcip
+                    };
+                    struct value value;
+                    int ret = bpf_map_lookup_elem(stats_fd, &next_key, &value);
+
+                    // Use key and value here
+
+                    cur_key = &next_key;
+            } while (next == 0);
+    }
-- 
2.35.1