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diff --git a/Documentation/networking/device_drivers/ethernet/amazon/ena.rst b/Documentation/networking/device_drivers/ethernet/amazon/ena.rst new file mode 100644 index 0000000000..5eaa3ab6c7 --- /dev/null +++ b/Documentation/networking/device_drivers/ethernet/amazon/ena.rst @@ -0,0 +1,351 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================================================ +Linux kernel driver for Elastic Network Adapter (ENA) family +============================================================ + +Overview +======== + +ENA is a networking interface designed to make good use of modern CPU +features and system architectures. + +The ENA device exposes a lightweight management interface with a +minimal set of memory mapped registers and extendible command set +through an Admin Queue. + +The driver supports a range of ENA devices, is link-speed independent +(i.e., the same driver is used for 10GbE, 25GbE, 40GbE, etc), and has +a negotiated and extendible feature set. + +Some ENA devices support SR-IOV. This driver is used for both the +SR-IOV Physical Function (PF) and Virtual Function (VF) devices. + +ENA devices enable high speed and low overhead network traffic +processing by providing multiple Tx/Rx queue pairs (the maximum number +is advertised by the device via the Admin Queue), a dedicated MSI-X +interrupt vector per Tx/Rx queue pair, adaptive interrupt moderation, +and CPU cacheline optimized data placement. + +The ENA driver supports industry standard TCP/IP offload features such as +checksum offload. Receive-side scaling (RSS) is supported for multi-core +scaling. + +The ENA driver and its corresponding devices implement health +monitoring mechanisms such as watchdog, enabling the device and driver +to recover in a manner transparent to the application, as well as +debug logs. + +Some of the ENA devices support a working mode called Low-latency +Queue (LLQ), which saves several more microseconds. + +ENA Source Code Directory Structure +=================================== + +================= ====================================================== +ena_com.[ch] Management communication layer. This layer is + responsible for the handling all the management + (admin) communication between the device and the + driver. +ena_eth_com.[ch] Tx/Rx data path. +ena_admin_defs.h Definition of ENA management interface. +ena_eth_io_defs.h Definition of ENA data path interface. +ena_common_defs.h Common definitions for ena_com layer. +ena_regs_defs.h Definition of ENA PCI memory-mapped (MMIO) registers. +ena_netdev.[ch] Main Linux kernel driver. +ena_ethtool.c ethtool callbacks. +ena_pci_id_tbl.h Supported device IDs. +================= ====================================================== + +Management Interface: +===================== + +ENA management interface is exposed by means of: + +- PCIe Configuration Space +- Device Registers +- Admin Queue (AQ) and Admin Completion Queue (ACQ) +- Asynchronous Event Notification Queue (AENQ) + +ENA device MMIO Registers are accessed only during driver +initialization and are not used during further normal device +operation. + +AQ is used for submitting management commands, and the +results/responses are reported asynchronously through ACQ. + +ENA introduces a small set of management commands with room for +vendor-specific extensions. Most of the management operations are +framed in a generic Get/Set feature command. + +The following admin queue commands are supported: + +- Create I/O submission queue +- Create I/O completion queue +- Destroy I/O submission queue +- Destroy I/O completion queue +- Get feature +- Set feature +- Configure AENQ +- Get statistics + +Refer to ena_admin_defs.h for the list of supported Get/Set Feature +properties. + +The Asynchronous Event Notification Queue (AENQ) is a uni-directional +queue used by the ENA device to send to the driver events that cannot +be reported using ACQ. AENQ events are subdivided into groups. Each +group may have multiple syndromes, as shown below + +The events are: + +==================== =============== +Group Syndrome +==================== =============== +Link state change **X** +Fatal error **X** +Notification Suspend traffic +Notification Resume traffic +Keep-Alive **X** +==================== =============== + +ACQ and AENQ share the same MSI-X vector. + +Keep-Alive is a special mechanism that allows monitoring the device's health. +A Keep-Alive event is delivered by the device every second. +The driver maintains a watchdog (WD) handler which logs the current state and +statistics. If the keep-alive events aren't delivered as expected the WD resets +the device and the driver. + +Data Path Interface +=================== + +I/O operations are based on Tx and Rx Submission Queues (Tx SQ and Rx +SQ correspondingly). Each SQ has a completion queue (CQ) associated +with it. + +The SQs and CQs are implemented as descriptor rings in contiguous +physical memory. + +The ENA driver supports two Queue Operation modes for Tx SQs: + +- **Regular mode:** + In this mode the Tx SQs reside in the host's memory. The ENA + device fetches the ENA Tx descriptors and packet data from host + memory. + +- **Low Latency Queue (LLQ) mode or "push-mode":** + In this mode the driver pushes the transmit descriptors and the + first 96 bytes of the packet directly to the ENA device memory + space. The rest of the packet payload is fetched by the + device. For this operation mode, the driver uses a dedicated PCI + device memory BAR, which is mapped with write-combine capability. + + **Note that** not all ENA devices support LLQ, and this feature is negotiated + with the device upon initialization. If the ENA device does not + support LLQ mode, the driver falls back to the regular mode. + +The Rx SQs support only the regular mode. + +The driver supports multi-queue for both Tx and Rx. This has various +benefits: + +- Reduced CPU/thread/process contention on a given Ethernet interface. +- Cache miss rate on completion is reduced, particularly for data + cache lines that hold the sk_buff structures. +- Increased process-level parallelism when handling received packets. +- Increased data cache hit rate, by steering kernel processing of + packets to the CPU, where the application thread consuming the + packet is running. +- In hardware interrupt re-direction. + +Interrupt Modes +=============== + +The driver assigns a single MSI-X vector per queue pair (for both Tx +and Rx directions). The driver assigns an additional dedicated MSI-X vector +for management (for ACQ and AENQ). + +Management interrupt registration is performed when the Linux kernel +probes the adapter, and it is de-registered when the adapter is +removed. I/O queue interrupt registration is performed when the Linux +interface of the adapter is opened, and it is de-registered when the +interface is closed. + +The management interrupt is named:: + + ena-mgmnt@pci:<PCI domain:bus:slot.function> + +and for each queue pair, an interrupt is named:: + + <interface name>-Tx-Rx-<queue index> + +The ENA device operates in auto-mask and auto-clear interrupt +modes. That is, once MSI-X is delivered to the host, its Cause bit is +automatically cleared and the interrupt is masked. The interrupt is +unmasked by the driver after NAPI processing is complete. + +Interrupt Moderation +==================== + +ENA driver and device can operate in conventional or adaptive interrupt +moderation mode. + +**In conventional mode** the driver instructs device to postpone interrupt +posting according to static interrupt delay value. The interrupt delay +value can be configured through `ethtool(8)`. The following `ethtool` +parameters are supported by the driver: ``tx-usecs``, ``rx-usecs`` + +**In adaptive interrupt** moderation mode the interrupt delay value is +updated by the driver dynamically and adjusted every NAPI cycle +according to the traffic nature. + +Adaptive coalescing can be switched on/off through `ethtool(8)`'s +:code:`adaptive_rx on|off` parameter. + +More information about Adaptive Interrupt Moderation (DIM) can be found in +Documentation/networking/net_dim.rst + +.. _`RX copybreak`: + +RX copybreak +============ +The rx_copybreak is initialized by default to ENA_DEFAULT_RX_COPYBREAK +and can be configured by the ETHTOOL_STUNABLE command of the +SIOCETHTOOL ioctl. + +Statistics +========== + +The user can obtain ENA device and driver statistics using `ethtool`. +The driver can collect regular or extended statistics (including +per-queue stats) from the device. + +In addition the driver logs the stats to syslog upon device reset. + +MTU +=== + +The driver supports an arbitrarily large MTU with a maximum that is +negotiated with the device. The driver configures MTU using the +SetFeature command (ENA_ADMIN_MTU property). The user can change MTU +via `ip(8)` and similar legacy tools. + +Stateless Offloads +================== + +The ENA driver supports: + +- IPv4 header checksum offload +- TCP/UDP over IPv4/IPv6 checksum offloads + +RSS +=== + +- The ENA device supports RSS that allows flexible Rx traffic + steering. +- Toeplitz and CRC32 hash functions are supported. +- Different combinations of L2/L3/L4 fields can be configured as + inputs for hash functions. +- The driver configures RSS settings using the AQ SetFeature command + (ENA_ADMIN_RSS_HASH_FUNCTION, ENA_ADMIN_RSS_HASH_INPUT and + ENA_ADMIN_RSS_INDIRECTION_TABLE_CONFIG properties). +- If the NETIF_F_RXHASH flag is set, the 32-bit result of the hash + function delivered in the Rx CQ descriptor is set in the received + SKB. +- The user can provide a hash key, hash function, and configure the + indirection table through `ethtool(8)`. + +DATA PATH +========= + +Tx +-- + +:code:`ena_start_xmit()` is called by the stack. This function does the following: + +- Maps data buffers (``skb->data`` and frags). +- Populates ``ena_buf`` for the push buffer (if the driver and device are + in push mode). +- Prepares ENA bufs for the remaining frags. +- Allocates a new request ID from the empty ``req_id`` ring. The request + ID is the index of the packet in the Tx info. This is used for + out-of-order Tx completions. +- Adds the packet to the proper place in the Tx ring. +- Calls :code:`ena_com_prepare_tx()`, an ENA communication layer that converts + the ``ena_bufs`` to ENA descriptors (and adds meta ENA descriptors as + needed). + + * This function also copies the ENA descriptors and the push buffer + to the Device memory space (if in push mode). + +- Writes a doorbell to the ENA device. +- When the ENA device finishes sending the packet, a completion + interrupt is raised. +- The interrupt handler schedules NAPI. +- The :code:`ena_clean_tx_irq()` function is called. This function handles the + completion descriptors generated by the ENA, with a single + completion descriptor per completed packet. + + * ``req_id`` is retrieved from the completion descriptor. The ``tx_info`` of + the packet is retrieved via the ``req_id``. The data buffers are + unmapped and ``req_id`` is returned to the empty ``req_id`` ring. + * The function stops when the completion descriptors are completed or + the budget is reached. + +Rx +-- + +- When a packet is received from the ENA device. +- The interrupt handler schedules NAPI. +- The :code:`ena_clean_rx_irq()` function is called. This function calls + :code:`ena_com_rx_pkt()`, an ENA communication layer function, which returns the + number of descriptors used for a new packet, and zero if + no new packet is found. +- :code:`ena_rx_skb()` checks packet length: + + * If the packet is small (len < rx_copybreak), the driver allocates + a SKB for the new packet, and copies the packet payload into the + SKB data buffer. + + - In this way the original data buffer is not passed to the stack + and is reused for future Rx packets. + + * Otherwise the function unmaps the Rx buffer, sets the first + descriptor as `skb`'s linear part and the other descriptors as the + `skb`'s frags. + +- The new SKB is updated with the necessary information (protocol, + checksum hw verify result, etc), and then passed to the network + stack, using the NAPI interface function :code:`napi_gro_receive()`. + +Dynamic RX Buffers (DRB) +------------------------ + +Each RX descriptor in the RX ring is a single memory page (which is either 4KB +or 16KB long depending on system's configurations). +To reduce the memory allocations required when dealing with a high rate of small +packets, the driver tries to reuse the remaining RX descriptor's space if more +than 2KB of this page remain unused. + +A simple example of this mechanism is the following sequence of events: + +:: + + 1. Driver allocates page-sized RX buffer and passes it to hardware + +----------------------+ + |4KB RX Buffer | + +----------------------+ + + 2. A 300Bytes packet is received on this buffer + + 3. The driver increases the ref count on this page and returns it back to + HW as an RX buffer of size 4KB - 300Bytes = 3796 Bytes + +----+--------------------+ + |****|3796 Bytes RX Buffer| + +----+--------------------+ + +This mechanism isn't used when an XDP program is loaded, or when the +RX packet is less than rx_copybreak bytes (in which case the packet is +copied out of the RX buffer into the linear part of a new skb allocated +for it and the RX buffer remains the same size, see `RX copybreak`_). |