Documentation: networking: Add description for multi-pf netdev

Add documentation for the multi-pf netdev feature.
Describe the mlx5 implementation and design decisions.
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>

Documentation/networking/index.rst

@@ -74,6 +74,7 @@ Contents:
    mpls-sysctl
    mptcp-sysctl
    multiqueue
+   multi-pf-netdev
    napi
    net_cachelines/index
    netconsole

Documentation/networking/multi-pf-netdev.rst

+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+===============
+Multi-PF Netdev
+===============
+
+Contents
+========
+
+- `Background`_
+- `Overview`_
+- `mlx5 implementation`_
+- `Channels distribution`_
+- `Observability`_
+- `Steering`_
+- `Mutually exclusive features`_
+
+Background
+==========
+
+The Multi-PF NIC technology enables several CPUs within a multi-socket server to connect directly to
+the network, each through its own dedicated PCIe interface. Through either a connection harness that
+splits the PCIe lanes between two cards or by bifurcating a PCIe slot for a single card. This
+results in eliminating the network traffic traversing over the internal bus between the sockets,
+significantly reducing overhead and latency, in addition to reducing CPU utilization and increasing
+network throughput.
+
+Overview
+========
+
+The feature adds support for combining multiple PFs of the same port in a Multi-PF environment under
+one netdev instance. It is implemented in the netdev layer. Lower-layer instances like pci func,
+sysfs entry, and devlink are kept separate.
+Passing traffic through different devices belonging to different NUMA sockets saves cross-NUMA
+traffic and allows apps running on the same netdev from different NUMAs to still feel a sense of
+proximity to the device and achieve improved performance.
+
+mlx5 implementation
+===================
+
+Multi-PF or Socket-direct in mlx5 is achieved by grouping PFs together which belong to the same
+NIC and has the socket-direct property enabled, once all PFs are probed, we create a single netdev
+to represent all of them, symmetrically, we destroy the netdev whenever any of the PFs is removed.
+
+The netdev network channels are distributed between all devices, a proper configuration would utilize
+the correct close NUMA node when working on a certain app/CPU.
+
+We pick one PF to be a primary (leader), and it fills a special role. The other devices
+(secondaries) are disconnected from the network at the chip level (set to silent mode). In silent
+mode, no south <-> north traffic flowing directly through a secondary PF. It needs the assistance of
+the leader PF (east <-> west traffic) to function. All Rx/Tx traffic is steered through the primary
+to/from the secondaries.
+
+Currently, we limit the support to PFs only, and up to two PFs (sockets).
+
+Channels distribution
+=====================
+
+We distribute the channels between the different PFs to achieve local NUMA node performance
+on multiple NUMA nodes.
+
+Each combined channel works against one specific PF, creating all its datapath queues against it. We
+distribute channels to PFs in a round-robin policy.
+
+::
+
+        Example for 2 PFs and 5 channels:
+        +--------+--------+
+        | ch idx | PF idx |
+        +--------+--------+
+        |    0   |    0   |
+        |    1   |    1   |
+        |    2   |    0   |
+        |    3   |    1   |
+        |    4   |    0   |
+        +--------+--------+
+
+
+The reason we prefer round-robin is, it is less influenced by changes in the number of channels. The
+mapping between a channel index and a PF is fixed, no matter how many channels the user configures.
+As the channel stats are persistent across channel's closure, changing the mapping every single time
+would turn the accumulative stats less representing of the channel's history.
+
+This is achieved by using the correct core device instance (mdev) in each channel, instead of them
+all using the same instance under "priv->mdev".
+
+Observability
+=============
+The relation between PF, irq, napi, and queue can be observed via netlink spec:
+
+$ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/netdev.yaml --dump queue-get --json='{"ifindex": 13}'
+[{'id': 0, 'ifindex': 13, 'napi-id': 539, 'type': 'rx'},
+ {'id': 1, 'ifindex': 13, 'napi-id': 540, 'type': 'rx'},
+ {'id': 2, 'ifindex': 13, 'napi-id': 541, 'type': 'rx'},
+ {'id': 3, 'ifindex': 13, 'napi-id': 542, 'type': 'rx'},
+ {'id': 4, 'ifindex': 13, 'napi-id': 543, 'type': 'rx'},
+ {'id': 0, 'ifindex': 13, 'napi-id': 539, 'type': 'tx'},
+ {'id': 1, 'ifindex': 13, 'napi-id': 540, 'type': 'tx'},
+ {'id': 2, 'ifindex': 13, 'napi-id': 541, 'type': 'tx'},
+ {'id': 3, 'ifindex': 13, 'napi-id': 542, 'type': 'tx'},
+ {'id': 4, 'ifindex': 13, 'napi-id': 543, 'type': 'tx'}]
+
+$ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/netdev.yaml --dump napi-get --json='{"ifindex": 13}'
+[{'id': 543, 'ifindex': 13, 'irq': 42},
+ {'id': 542, 'ifindex': 13, 'irq': 41},
+ {'id': 541, 'ifindex': 13, 'irq': 40},
+ {'id': 540, 'ifindex': 13, 'irq': 39},
+ {'id': 539, 'ifindex': 13, 'irq': 36}]
+
+Here you can clearly observe our channels distribution policy:
+
+$ ls /proc/irq/{36,39,40,41,42}/mlx5* -d -1
+/proc/irq/36/mlx5_comp1@pci:0000:08:00.0
+/proc/irq/39/mlx5_comp1@pci:0000:09:00.0
+/proc/irq/40/mlx5_comp2@pci:0000:08:00.0
+/proc/irq/41/mlx5_comp2@pci:0000:09:00.0
+/proc/irq/42/mlx5_comp3@pci:0000:08:00.0
+
+Steering
+========
+Secondary PFs are set to "silent" mode, meaning they are disconnected from the network.
+
+In Rx, the steering tables belong to the primary PF only, and it is its role to distribute incoming
+traffic to other PFs, via cross-vhca steering capabilities. Still maintain a single default RSS table,
+that is capable of pointing to the receive queues of a different PF.
+
+In Tx, the primary PF creates a new Tx flow table, which is aliased by the secondaries, so they can
+go out to the network through it.
+
+In addition, we set default XPS configuration that, based on the CPU, selects an SQ belonging to the
+PF on the same node as the CPU.
+
+XPS default config example:
+
+NUMA node(s):          2
+NUMA node0 CPU(s):     0-11
+NUMA node1 CPU(s):     12-23
+
+PF0 on node0, PF1 on node1.
+
+- /sys/class/net/eth2/queues/tx-0/xps_cpus:000001
+- /sys/class/net/eth2/queues/tx-1/xps_cpus:001000
+- /sys/class/net/eth2/queues/tx-2/xps_cpus:000002
+- /sys/class/net/eth2/queues/tx-3/xps_cpus:002000
+- /sys/class/net/eth2/queues/tx-4/xps_cpus:000004
+- /sys/class/net/eth2/queues/tx-5/xps_cpus:004000
+- /sys/class/net/eth2/queues/tx-6/xps_cpus:000008
+- /sys/class/net/eth2/queues/tx-7/xps_cpus:008000
+- /sys/class/net/eth2/queues/tx-8/xps_cpus:000010
+- /sys/class/net/eth2/queues/tx-9/xps_cpus:010000
+- /sys/class/net/eth2/queues/tx-10/xps_cpus:000020
+- /sys/class/net/eth2/queues/tx-11/xps_cpus:020000
+- /sys/class/net/eth2/queues/tx-12/xps_cpus:000040
+- /sys/class/net/eth2/queues/tx-13/xps_cpus:040000
+- /sys/class/net/eth2/queues/tx-14/xps_cpus:000080
+- /sys/class/net/eth2/queues/tx-15/xps_cpus:080000
+- /sys/class/net/eth2/queues/tx-16/xps_cpus:000100
+- /sys/class/net/eth2/queues/tx-17/xps_cpus:100000
+- /sys/class/net/eth2/queues/tx-18/xps_cpus:000200
+- /sys/class/net/eth2/queues/tx-19/xps_cpus:200000
+- /sys/class/net/eth2/queues/tx-20/xps_cpus:000400
+- /sys/class/net/eth2/queues/tx-21/xps_cpus:400000
+- /sys/class/net/eth2/queues/tx-22/xps_cpus:000800
+- /sys/class/net/eth2/queues/tx-23/xps_cpus:800000
+
+Mutually exclusive features
+===========================
+
+The nature of Multi-PF, where different channels work with different PFs, conflicts with
+stateful features where the state is maintained in one of the PFs.
+For example, in the TLS device-offload feature, special context objects are created per connection
+and maintained in the PF.  Transitioning between different RQs/SQs would break the feature. Hence,
+we disable this combination for now.