--- /dev/null
+.. _TX_Queue_doc:
+
+Transmit Queues
+===============
+
+Overview
+________
+
+VPP implements Transmit queues infra to access and manage them. It provides
+common registration functions to register or unregister interfaces’ transmit
+queues. It also provides functions for queues placement on given thread(s).
+
+The TXQ Infrastructure
+_______________________
+
+Infra registers each queue using a unique key which is formed by concatenating
+the hardware interface index ``hw_if_index`` and unique queue identifier for
+given interface ``queue_id``. As a result of registration of queue, infra
+returns back a unique global ``queue_index`` which can be used by driver to
+access that queue later.
+
+Interface output node uses pre-computed ``output_node_thread_runtime`` data
+which provides essential information related to queue placements on given
+thread of given interface. Transmit queue infra implements an algorithm to
+pre-compute this information. It also pre-computes scalar arguments of frame
+``vnet_hw_if_tx_frame_t``. It also pre-calculates a ``lookup_table`` for
+thread if there are multiple transmit queues are placed on that thread.
+Interface drivers call ``vnet_hw_if_update_runtime_data()`` to execute that
+algorithm after registering the transmit queues to TXQ infra.
+
+The algorithm makes the copy of existing runtime data and iterate through them
+for each vpp main and worker thread. In each iteration, algorithm loop through
+all the tx queues of given interface to fill the information in the frame data
+structure ``vnet_hw_if_tx_frame_t``. Algorithm also updates the information
+related to number of transmit queues of given interface on given vpp thread in
+data structure ``output_node_thread_runtime``. As a consequence of any update
+to the copy, triggers the function to update the actual working copy by taking
+the worker barrier and free the old copy of ``output_node_thread_runtime``.
+
+Multi-TXQ infra
+^^^^^^^^^^^^^^^
+
+Interface output node uses packet flow hash using hash infra in case of multi-txq
+on given thread. Each hardware interface class contains type of the hash required
+for interfaces from that hardware interface class i.e. ethernet interface hardware
+class contains type ``VNET_HASH_FN_TYPE_ETHERNET``. Though, the hash function
+itself is contained by hardware interface data structure of given interface. Default
+hashing function is selected upon interface creation based on priority. User can
+configure a different hash to an interface for multi-txq use case.
+
+Interface output node uses packet flow hash as an index to the pre-calculated lookup
+table to get the queue identifier for given transmit queue. Interface output node
+enqueues the packets to respective frame and also copies the ``vnet_hw_if_tx_frame_t``
+to frame scalar arguments. Drivers use scalar arguments ``vnet_hw_if_tx_frame_t``
+of the given frame to extract the information about the transmit queue to be used to
+transmit the packets. Drivers may need to acquire a lock on given queue before
+transmitting the packets based on the ``shared_queue`` bit status.
+
+Data structures
+^^^^^^^^^^^^^^^
+
+Queue information is stored in data structure ``vnet_hw_if_tx_queue_t``:
+
+.. code:: c
+
+ typedef struct
+ {
+ /* either this queue is shared among multiple threads */
+ u8 shared_queue : 1;
+ /* hw interface index */
+ u32 hw_if_index;
+
+ /* hardware queue identifier */
+ u32 queue_id;
+
+ /* bitmap of threads which use this queue */
+ clib_bitmap_t *threads;
+ } vnet_hw_if_tx_queue_t;
+
+
+Frame information is stored in data structure: ``vnet_hw_if_tx_frame_t``:
+
+.. code:: c
+
+ typedef enum
+ {
+ VNET_HW_IF_TX_FRAME_HINT_NOT_CHAINED = (1 << 0),
+ VNET_HW_IF_TX_FRAME_HINT_NO_GSO = (1 << 1),
+ VNET_HW_IF_TX_FRAME_HINT_NO_CKSUM_OFFLOAD = (1 << 2),
+ } vnet_hw_if_tx_frame_hint_t;
+
+ typedef struct
+ {
+ u8 shared_queue : 1;
+ vnet_hw_if_tx_frame_hint_t hints : 16;
+ u32 queue_id;
+ } vnet_hw_if_tx_frame_t;
+
+Output node runtime information is stored in data structure: ``output_node_thread_runtime``:
+
+.. code:: c
+
+ typedef struct
+ {
+ CLIB_CACHE_LINE_ALIGN_MARK (cacheline0);
+ vnet_hw_if_tx_frame_t *frame;
+ u32 *lookup_table;
+ u32 n_queues;
+ } vnet_hw_if_output_node_runtime_t;
+
+
+MultiTXQ API
+^^^^^^^^^^^^
+
+This API message is used to place tx queue of an interface to vpp main or worker(s) thread(s).
+
+.. code:: c
+
+ autoendian autoreply define sw_interface_set_tx_placement
+ {
+ u32 client_index;
+ u32 context;
+ vl_api_interface_index_t sw_if_index;
+ u32 queue_id;
+ u32 array_size;
+ u32 threads[array_size];
+ option vat_help = "<interface | sw_if_index <index>> queue <n> [threads <list> | mask <hex>]";
+ };
+
+Multi-TXQ CLI
+^^^^^^^^^^^^^
+
+::
+
+ set interface tx-queue set interface tx-queue <interface> queue <n> [threads <list>]
+ set interface tx-hash set interface tx-hash <interface> hash-name <hash-name>
+
+::
+
+ show hardware-interfaces
+
+ Name Idx Link Hardware
+ tap0 1 up tap0
+ Link speed: unknown
+ RX Queues:
+ queue thread mode
+ 0 main (0) polling
+ TX Queues:
+ TX Hash: [name: crc32c-5tuple priority: 50 description: IPv4/IPv6 header and TCP/UDP ports]
+ queue shared thread(s)
+ 0 no 0
+ Ethernet address 02:fe:27:69:5a:b5
+ VIRTIO interface
+ instance 0
+ RX QUEUE : Total Packets
+ 0 : 0
+ TX QUEUE : Total Packets
+ 0 : 0
+
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