hash: add hash documentation

Type: docs

Signed-off-by: Mohsin Kazmi <sykazmi@cisco.com>
Change-Id: I36764aa3c0e9657d228413aeafa0a54bbd755e49

diff --git a/docs/developer/corefeatures/hash.rst b/docs/developer/corefeatures/hash.rst
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+../../../src/vnet/hash/hash.rst
\ No newline at end of file

diff --git a/docs/developer/corefeatures/index.rst b/docs/developer/corefeatures/index.rst
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--- a/docs/developer/corefeatures/index.rst
+++ b/docs/developer/corefeatures/index.rst
@@ -23,3 +23,4 @@ Core Features
     stats
     selinux_doc
     policer
+    hash

diff --git a/src/vnet/hash/hash.rst b/src/vnet/hash/hash.rst
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+.. _hash_doc:
+
+Hash Infra
+==========
+
+Overview
+________
+
+Modern physical NICs uses packet flow hash for different purposes, i.e. Receive
+Side Scaling, flow steering and interface bonding etc. NICs can also provide
+packet flow hash prepended to data packet as metadata which can be used by
+applications without recomputing the packet flow hash.
+
+As more and more services are deployed in virtualized environment, making use of
+virtual interfaces to interconnect those services.
+
+The Hash Infrastructure
+_______________________
+
+VPP implements software based hashing functionality which can be used for different
+purposes. It also provides users a centralized way to registry custom hash functions
+based on traffic profile to be used in different vpp features i.e. Multi-TXQ,
+software RSS or bonding driver.
+
+Data structures
+^^^^^^^^^^^^^^^
+
+Hashing infra provides two types of hashing functions:
+``VNET_HASH_FN_TYPE_ETHERNET`` and ``VNET_HASH_FN_TYPE_IP`` for ethernet traffic and
+IP traffic respectively.
+Hashing infra provides uniform signature to the functions to be implemented:
+
+.. code:: c
+
+  void (*vnet_hash_fn_t) (void **p, u32 *h, u32 n_packets);
+
+Here ``**p`` is the array of pointers pointing to the beginning of packet headers
+(either ethernet or ip).
+``*h`` is an empty array of size n_packets. On return, it will contain hashes.
+``n_packets`` is the number of packets pass to this function.
+
+Custom hashing functions can be registered through ``VNET_REGISTER_HASH_FUNCTION``.
+Users need to provide a name, description, priority and hashing functions for
+registration.
+
+Default hashing function is selected based on the highest priority among the registered
+hashing functions.
+
+.. code:: c
+
+  typedef struct vnet_hash_function_registration
+  {
+    const char *name;
+    const char *description;
+    int priority;
+    vnet_hash_fn_t function[VNET_HASH_FN_TYPE_N];
+
+    struct vnet_hash_function_registration *next;
+  } vnet_hash_function_registration_t;
+
+For example, ``crc32c_5tuple`` provides two hashing functions: for IP traffic and for
+ethernet traffic. It uses 5 tuples from the flow to compute the crc32 hash on it.
+
+.. code:: c
+
+  void vnet_crc32c_5tuple_ip_func (void **p, u32 *hash, u32 n_packets);
+  void vnet_crc32c_5tuple_ethernet_func (void **p, u32 *hash, u32 n_packets);
+
+  VNET_REGISTER_HASH_FUNCTION (crc32c_5tuple, static) = {
+    .name = "crc32c-5tuple",
+    .description = "IPv4/IPv6 header and TCP/UDP ports",
+    .priority = 50,
+    .function[VNET_HASH_FN_TYPE_ETHERNET] = vnet_crc32c_5tuple_ethernet_func,
+    .function[VNET_HASH_FN_TYPE_IP] = vnet_crc32c_5tuple_ip_func,
+  };
+
+
+Users can see all the registered hash functions along with priority and description.
+
+Hash API
+^^^^^^^^
+
+There is no Hash API at the moment.
+
+Hash CLI
+^^^^^^^^
+
+::
+
+  show hash