devlink: introduce devlink-dpipe.rst documentation file

Primarily based on the DPIPE netdev conference paper, introduce a new
file to document the dpipe interface.

This likely needs further improvement, but is at least a good overall
start.

Signed-off-by: Jacob Keller <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/devlink/devlink-dpipe.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+Devlink DPIPE
+=============
+
+Background
+==========
+
+While performing the hardware offloading process, much of the hardware
+specifics cannot be presented. These details are useful for debugging, and
+``devlink-dpipe`` provides a standardized way to provide visibility into the
+offloading process.
+
+For example, the routing longest prefix match (LPM) algorithm used by the
+Linux kernel may differ from the hardware implementation. The pipeline debug
+API (DPIPE) is aimed at providing the user visibility into the ASIC's
+pipeline in a generic way.
+
+The hardware offload process is expected to be done in a way that the user
+should not be able to distinguish between the hardware vs. software
+implementation. In this process, hardware specifics are neglected. In
+reality those details can have lots of meaning and should be exposed in some
+standard way.
+
+This problem is made even more complex when one wishes to offload the
+control path of the whole networking stack to a switch ASIC. Due to
+differences in the hardware and software models some processes cannot be
+represented correctly.
+
+One example is the kernel's LPM algorithm which in many cases differs
+greatly to the hardware implementation. The configuration API is the same,
+but one cannot rely on the Forward Information Base (FIB) to look like the
+Level Path Compression trie (LPC-trie) in hardware.
+
+In many situations trying to analyze systems failure solely based on the
+kernel's dump may not be enough. By combining this data with complementary
+information about the underlying hardware, this debugging can be made
+easier; additionally, the information can be useful when debugging
+performance issues.
+
+Overview
+========
+
+The ``devlink-dpipe`` interface closes this gap. The hardware's pipeline is
+modeled as a graph of match/action tables. Each table represents a specific
+hardware block. This model is not new, first being used by the P4 language.
+
+Traditionally it has been used as an alternative model for hardware
+configuration, but the ``devlink-dpipe`` interface uses it for visibility
+purposes as a standard complementary tool. The system's view from
+``devlink-dpipe`` should change according to the changes done by the
+standard configuration tools.
+
+For example, it’s quiet common to  implement Access Control Lists (ACL)
+using Ternary Content Addressable Memory (TCAM). The TCAM memory can be
+divided into TCAM regions. Complex TC filters can have multiple rules with
+different priorities and different lookup keys. On the other hand hardware
+TCAM regions have a predefined lookup key. Offloading the TC filter rules
+using TCAM engine can result in multiple TCAM regions being interconnected
+in a chain (which may affect the data path latency). In response to a new TC
+filter new tables should be created describing those regions.
+
+Model
+=====
+
+The ``DPIPE`` model introduces several objects:
+
+  * headers
+  * tables
+  * entries
+
+A ``header`` describes packet formats and provides names for fields within
+the packet. A ``table`` describes hardware blocks. An ``entry`` describes
+the actual content of a specific table.
+
+The hardware pipeline is not port specific, but rather describes the whole
+ASIC. Thus it is tied to the top of the ``devlink`` infrastructure.
+
+Drivers can register and unregister tables at run time, in order to support
+dynamic behavior. This dynamic behavior is mandatory for describing hardware
+blocks like TCAM regions which can be allocated and freed dynamically.
+
+``devlink-dpipe`` generally is not intended for configuration. The exception
+is hardware counting for a specific table.
+
+The following commands are used to obtain the ``dpipe`` objects from
+userspace:
+
+  * ``table_get``: Receive a table's description.
+  * ``headers_get``: Receive a device's supported headers.
+  * ``entries_get``: Receive a table's current entries.
+  * ``counters_set``: Enable or disable counters on a table.
+
+Table
+-----
+
+The driver should implement the following operations for each table:
+
+  * ``matches_dump``: Dump the supported matches.
+  * ``actions_dump``: Dump the supported actions.
+  * ``entries_dump``: Dump the actual content of the table.
+  * ``counters_set_update``: Synchronize hardware with counters enabled or
+    disabled.
+
+Header/Field
+------------
+
+In a similar way to P4 headers and fields are used to describe a table's
+behavior. There is a slight difference between the standard protocol headers
+and specific ASIC metadata. The protocol headers should be declared in the
+``devlink`` core API. On the other hand ASIC meta data is driver specific
+and should be defined in the driver. Additionally, each driver-specific
+devlink documentation file should document the driver-specific ``dpipe``
+headers it implements. The headers and fields are identified by enumeration.
+
+In order to provide further visibility some ASIC metadata fields could be
+mapped to kernel objects. For example, internal router interface indexes can
+be directly mapped to the net device ifindex. FIB table indexes used by
+different Virtual Routing and Forwarding (VRF) tables can be mapped to
+internal routing table indexes.
+
+Match
+-----
+
+Matches are kept primitive and close to hardware operation. Match types like
+LPM are not supported due to the fact that this is exactly a process we wish
+to describe in full detail. Example of matches:
+
+  * ``field_exact``: Exact match on a specific field.
+  * ``field_exact_mask``: Exact match on a specific field after masking.
+  * ``field_range``: Match on a specific range.
+
+The id's of the header and the field should be specified in order to
+identify the specific field. Furthermore, the header index should be
+specified in order to distinguish multiple headers of the same type in a
+packet (tunneling).
+
+Action
+------
+
+Similar to match, the actions are kept primitive and close to hardware
+operation. For example:
+
+  * ``field_modify``: Modify the field value.
+  * ``field_inc``: Increment the field value.
+  * ``push_header``: Add a header.
+  * ``pop_header``: Remove a header.
+
+Entry
+-----
+
+Entries of a specific table can be dumped on demand. Each eentry is
+identified with an index and its properties are described by a list of
+match/action values and specific counter. By dumping the tables content the
+interactions between tables can be resolved.
+
+Abstraction Example
+===================
+
+The following is an example of the abstraction model of the L3 part of
+Mellanox Spectrum ASIC. The blocks are described in the order they appear in
+the pipeline. The table sizes in the following examples are not real
+hardware sizes and are provided for demonstration purposes.
+
+LPM
+---
+
+The LPM algorithm can be implemented as a list of hash tables. Each hash
+table contains routes with the same prefix length. The root of the list is
+/32, and in case of a miss the hardware will continue to the next hash
+table. The depth of the search will affect the data path latency.
+
+In case of a hit the entry contains information about the next stage of the
+pipeline which resolves the MAC address. The next stage can be either local
+host table for directly connected routes, or adjacency table for next-hops.
+The ``meta.lpm_prefix`` field is used to connect two LPM tables.
+
+.. code::
+
+    table lpm_prefix_16 {
+      size: 4096,
+      counters_enabled: true,
+      match: { meta.vr_id: exact,
+               ipv4.dst_addr: exact_mask,
+               ipv6.dst_addr: exact_mask,
+               meta.lpm_prefix: exact },
+      action: { meta.adj_index: set,
+                meta.adj_group_size: set,
+                meta.rif_port: set,
+                meta.lpm_prefix: set },
+    }
+
+Local Host
+----------
+
+In the case of local routes the LPM lookup already resolves the egress
+router interface (RIF), yet the exact MAC address is not known. The local
+host table is a hash table combining the output interface id with
+destination IP address as a key. The result is the MAC address.
+
+.. code::
+
+    table local_host {
+      size: 4096,
+      counters_enabled: true,
+      match: { meta.rif_port: exact,
+               ipv4.dst_addr: exact},
+      action: { ethernet.daddr: set }
+    }
+
+Adjacency
+---------
+
+In case of remote routes this table does the ECMP. The LPM lookup results in
+ECMP group size and index that serves as a global offset into this table.
+Concurrently a hash of the packet is generated. Based on the ECMP group size
+and the packet's hash a local offset is generated. Multiple LPM entries can
+point to the same adjacency group.
+
+.. code::
+
+    table adjacency {
+      size: 4096,
+      counters_enabled: true,
+      match: { meta.adj_index: exact,
+               meta.adj_group_size: exact,
+               meta.packet_hash_index: exact },
+      action: { ethernet.daddr: set,
+                meta.erif: set }
+    }
+
+ERIF
+----
+
+In case the egress RIF and destination MAC have been resolved by previous
+tables this table does multiple operations like TTL decrease and MTU check.
+Then the decision of forward/drop is taken and the port L3 statistics are
+updated based on the packet's type (broadcast, unicast, multicast).
+
+.. code::
+
+    table erif {
+      size: 800,
+      counters_enabled: true,
+      match: { meta.rif_port: exact,
+               meta.is_l3_unicast: exact,
+               meta.is_l3_broadcast: exact,
+               meta.is_l3_multicast, exact },
+      action: { meta.l3_drop: set,
+                meta.l3_forward: set }
+    }

Documentation/networking/devlink/index.rst

···
 .. toctree::
    :maxdepth: 1
 
+   devlink-dpipe
    devlink-health
    devlink-info
    devlink-params