1.. SPDX-License-Identifier: GPL-2.0 2 3============================================================ 4Linux kernel driver for Elastic Network Adapter (ENA) family 5============================================================ 6 7Overview 8======== 9 10ENA is a networking interface designed to make good use of modern CPU 11features and system architectures. 12 13The ENA device exposes a lightweight management interface with a 14minimal set of memory mapped registers and extendible command set 15through an Admin Queue. 16 17The driver supports a range of ENA devices, is link-speed independent 18(i.e., the same driver is used for 10GbE, 25GbE, 40GbE, etc), and has 19a negotiated and extendible feature set. 20 21Some ENA devices support SR-IOV. This driver is used for both the 22SR-IOV Physical Function (PF) and Virtual Function (VF) devices. 23 24ENA devices enable high speed and low overhead network traffic 25processing by providing multiple Tx/Rx queue pairs (the maximum number 26is advertised by the device via the Admin Queue), a dedicated MSI-X 27interrupt vector per Tx/Rx queue pair, adaptive interrupt moderation, 28and CPU cacheline optimized data placement. 29 30The ENA driver supports industry standard TCP/IP offload features such as 31checksum offload. Receive-side scaling (RSS) is supported for multi-core 32scaling. 33 34The ENA driver and its corresponding devices implement health 35monitoring mechanisms such as watchdog, enabling the device and driver 36to recover in a manner transparent to the application, as well as 37debug logs. 38 39Some of the ENA devices support a working mode called Low-latency 40Queue (LLQ), which saves several more microseconds. 41 42ENA Source Code Directory Structure 43=================================== 44 45================= ====================================================== 46ena_com.[ch] Management communication layer. This layer is 47 responsible for the handling all the management 48 (admin) communication between the device and the 49 driver. 50ena_eth_com.[ch] Tx/Rx data path. 51ena_admin_defs.h Definition of ENA management interface. 52ena_eth_io_defs.h Definition of ENA data path interface. 53ena_common_defs.h Common definitions for ena_com layer. 54ena_regs_defs.h Definition of ENA PCI memory-mapped (MMIO) registers. 55ena_netdev.[ch] Main Linux kernel driver. 56ena_ethtool.c ethtool callbacks. 57ena_xdp.[ch] XDP files 58ena_pci_id_tbl.h Supported device IDs. 59================= ====================================================== 60 61Management Interface: 62===================== 63 64ENA management interface is exposed by means of: 65 66- PCIe Configuration Space 67- Device Registers 68- Admin Queue (AQ) and Admin Completion Queue (ACQ) 69- Asynchronous Event Notification Queue (AENQ) 70 71ENA device MMIO Registers are accessed only during driver 72initialization and are not used during further normal device 73operation. 74 75AQ is used for submitting management commands, and the 76results/responses are reported asynchronously through ACQ. 77 78ENA introduces a small set of management commands with room for 79vendor-specific extensions. Most of the management operations are 80framed in a generic Get/Set feature command. 81 82The following admin queue commands are supported: 83 84- Create I/O submission queue 85- Create I/O completion queue 86- Destroy I/O submission queue 87- Destroy I/O completion queue 88- Get feature 89- Set feature 90- Configure AENQ 91- Get statistics 92 93Refer to ena_admin_defs.h for the list of supported Get/Set Feature 94properties. 95 96The Asynchronous Event Notification Queue (AENQ) is a uni-directional 97queue used by the ENA device to send to the driver events that cannot 98be reported using ACQ. AENQ events are subdivided into groups. Each 99group may have multiple syndromes, as shown below 100 101The events are: 102 103==================== =============== 104Group Syndrome 105==================== =============== 106Link state change **X** 107Fatal error **X** 108Notification Suspend traffic 109Notification Resume traffic 110Keep-Alive **X** 111==================== =============== 112 113ACQ and AENQ share the same MSI-X vector. 114 115Keep-Alive is a special mechanism that allows monitoring the device's health. 116A Keep-Alive event is delivered by the device every second. 117The driver maintains a watchdog (WD) handler which logs the current state and 118statistics. If the keep-alive events aren't delivered as expected the WD resets 119the device and the driver. 120 121Data Path Interface 122=================== 123 124I/O operations are based on Tx and Rx Submission Queues (Tx SQ and Rx 125SQ correspondingly). Each SQ has a completion queue (CQ) associated 126with it. 127 128The SQs and CQs are implemented as descriptor rings in contiguous 129physical memory. 130 131The ENA driver supports two Queue Operation modes for Tx SQs: 132 133- **Regular mode:** 134 In this mode the Tx SQs reside in the host's memory. The ENA 135 device fetches the ENA Tx descriptors and packet data from host 136 memory. 137 138- **Low Latency Queue (LLQ) mode or "push-mode":** 139 In this mode the driver pushes the transmit descriptors and the 140 first 96 bytes of the packet directly to the ENA device memory 141 space. The rest of the packet payload is fetched by the 142 device. For this operation mode, the driver uses a dedicated PCI 143 device memory BAR, which is mapped with write-combine capability. 144 145 **Note that** not all ENA devices support LLQ, and this feature is negotiated 146 with the device upon initialization. If the ENA device does not 147 support LLQ mode, the driver falls back to the regular mode. 148 149The Rx SQs support only the regular mode. 150 151The driver supports multi-queue for both Tx and Rx. This has various 152benefits: 153 154- Reduced CPU/thread/process contention on a given Ethernet interface. 155- Cache miss rate on completion is reduced, particularly for data 156 cache lines that hold the sk_buff structures. 157- Increased process-level parallelism when handling received packets. 158- Increased data cache hit rate, by steering kernel processing of 159 packets to the CPU, where the application thread consuming the 160 packet is running. 161- In hardware interrupt re-direction. 162 163Interrupt Modes 164=============== 165 166The driver assigns a single MSI-X vector per queue pair (for both Tx 167and Rx directions). The driver assigns an additional dedicated MSI-X vector 168for management (for ACQ and AENQ). 169 170Management interrupt registration is performed when the Linux kernel 171probes the adapter, and it is de-registered when the adapter is 172removed. I/O queue interrupt registration is performed when the Linux 173interface of the adapter is opened, and it is de-registered when the 174interface is closed. 175 176The management interrupt is named:: 177 178 ena-mgmnt@pci:<PCI domain:bus:slot.function> 179 180and for each queue pair, an interrupt is named:: 181 182 <interface name>-Tx-Rx-<queue index> 183 184The ENA device operates in auto-mask and auto-clear interrupt 185modes. That is, once MSI-X is delivered to the host, its Cause bit is 186automatically cleared and the interrupt is masked. The interrupt is 187unmasked by the driver after NAPI processing is complete. 188 189Interrupt Moderation 190==================== 191 192ENA driver and device can operate in conventional or adaptive interrupt 193moderation mode. 194 195**In conventional mode** the driver instructs device to postpone interrupt 196posting according to static interrupt delay value. The interrupt delay 197value can be configured through `ethtool(8)`. The following `ethtool` 198parameters are supported by the driver: ``tx-usecs``, ``rx-usecs`` 199 200**In adaptive interrupt** moderation mode the interrupt delay value is 201updated by the driver dynamically and adjusted every NAPI cycle 202according to the traffic nature. 203 204Adaptive coalescing can be switched on/off through `ethtool(8)`'s 205:code:`adaptive_rx on|off` parameter. 206 207More information about Adaptive Interrupt Moderation (DIM) can be found in 208Documentation/networking/net_dim.rst 209 210.. _`RX copybreak`: 211 212RX copybreak 213============ 214The rx_copybreak is initialized by default to ENA_DEFAULT_RX_COPYBREAK 215and can be configured by the ETHTOOL_STUNABLE command of the 216SIOCETHTOOL ioctl. 217 218Statistics 219========== 220 221The user can obtain ENA device and driver statistics using `ethtool`. 222The driver can collect regular or extended statistics (including 223per-queue stats) from the device. 224 225In addition the driver logs the stats to syslog upon device reset. 226 227MTU 228=== 229 230The driver supports an arbitrarily large MTU with a maximum that is 231negotiated with the device. The driver configures MTU using the 232SetFeature command (ENA_ADMIN_MTU property). The user can change MTU 233via `ip(8)` and similar legacy tools. 234 235Stateless Offloads 236================== 237 238The ENA driver supports: 239 240- IPv4 header checksum offload 241- TCP/UDP over IPv4/IPv6 checksum offloads 242 243RSS 244=== 245 246- The ENA device supports RSS that allows flexible Rx traffic 247 steering. 248- Toeplitz and CRC32 hash functions are supported. 249- Different combinations of L2/L3/L4 fields can be configured as 250 inputs for hash functions. 251- The driver configures RSS settings using the AQ SetFeature command 252 (ENA_ADMIN_RSS_HASH_FUNCTION, ENA_ADMIN_RSS_HASH_INPUT and 253 ENA_ADMIN_RSS_INDIRECTION_TABLE_CONFIG properties). 254- If the NETIF_F_RXHASH flag is set, the 32-bit result of the hash 255 function delivered in the Rx CQ descriptor is set in the received 256 SKB. 257- The user can provide a hash key, hash function, and configure the 258 indirection table through `ethtool(8)`. 259 260DATA PATH 261========= 262 263Tx 264-- 265 266:code:`ena_start_xmit()` is called by the stack. This function does the following: 267 268- Maps data buffers (``skb->data`` and frags). 269- Populates ``ena_buf`` for the push buffer (if the driver and device are 270 in push mode). 271- Prepares ENA bufs for the remaining frags. 272- Allocates a new request ID from the empty ``req_id`` ring. The request 273 ID is the index of the packet in the Tx info. This is used for 274 out-of-order Tx completions. 275- Adds the packet to the proper place in the Tx ring. 276- Calls :code:`ena_com_prepare_tx()`, an ENA communication layer that converts 277 the ``ena_bufs`` to ENA descriptors (and adds meta ENA descriptors as 278 needed). 279 280 * This function also copies the ENA descriptors and the push buffer 281 to the Device memory space (if in push mode). 282 283- Writes a doorbell to the ENA device. 284- When the ENA device finishes sending the packet, a completion 285 interrupt is raised. 286- The interrupt handler schedules NAPI. 287- The :code:`ena_clean_tx_irq()` function is called. This function handles the 288 completion descriptors generated by the ENA, with a single 289 completion descriptor per completed packet. 290 291 * ``req_id`` is retrieved from the completion descriptor. The ``tx_info`` of 292 the packet is retrieved via the ``req_id``. The data buffers are 293 unmapped and ``req_id`` is returned to the empty ``req_id`` ring. 294 * The function stops when the completion descriptors are completed or 295 the budget is reached. 296 297Rx 298-- 299 300- When a packet is received from the ENA device. 301- The interrupt handler schedules NAPI. 302- The :code:`ena_clean_rx_irq()` function is called. This function calls 303 :code:`ena_com_rx_pkt()`, an ENA communication layer function, which returns the 304 number of descriptors used for a new packet, and zero if 305 no new packet is found. 306- :code:`ena_rx_skb()` checks packet length: 307 308 * If the packet is small (len < rx_copybreak), the driver allocates 309 a SKB for the new packet, and copies the packet payload into the 310 SKB data buffer. 311 312 - In this way the original data buffer is not passed to the stack 313 and is reused for future Rx packets. 314 315 * Otherwise the function unmaps the Rx buffer, sets the first 316 descriptor as `skb`'s linear part and the other descriptors as the 317 `skb`'s frags. 318 319- The new SKB is updated with the necessary information (protocol, 320 checksum hw verify result, etc), and then passed to the network 321 stack, using the NAPI interface function :code:`napi_gro_receive()`. 322 323Dynamic RX Buffers (DRB) 324------------------------ 325 326Each RX descriptor in the RX ring is a single memory page (which is either 4KB 327or 16KB long depending on system's configurations). 328To reduce the memory allocations required when dealing with a high rate of small 329packets, the driver tries to reuse the remaining RX descriptor's space if more 330than 2KB of this page remain unused. 331 332A simple example of this mechanism is the following sequence of events: 333 334:: 335 336 1. Driver allocates page-sized RX buffer and passes it to hardware 337 +----------------------+ 338 |4KB RX Buffer | 339 +----------------------+ 340 341 2. A 300Bytes packet is received on this buffer 342 343 3. The driver increases the ref count on this page and returns it back to 344 HW as an RX buffer of size 4KB - 300Bytes = 3796 Bytes 345 +----+--------------------+ 346 |****|3796 Bytes RX Buffer| 347 +----+--------------------+ 348 349This mechanism isn't used when an XDP program is loaded, or when the 350RX packet is less than rx_copybreak bytes (in which case the packet is 351copied out of the RX buffer into the linear part of a new skb allocated 352for it and the RX buffer remains the same size, see `RX copybreak`_). 353