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diff --git a/Documentation/networking/devlink/devlink-dpipe.rst b/Documentation/networking/devlink/devlink-dpipe.rst new file mode 100644 index 000000000..af37f250d --- /dev/null +++ b/Documentation/networking/devlink/devlink-dpipe.rst @@ -0,0 +1,252 @@ +.. 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 quite 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 } + } |