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============ 214 215The rx_copybreak is initialized by default to ENA_DEFAULT_RX_COPYBREAK 216and can be configured by the ETHTOOL_STUNABLE command of the 217SIOCETHTOOL ioctl. 218 219This option controls the maximum packet length for which the RX 220descriptor it was received on would be recycled. When a packet smaller 221than RX copybreak bytes is received, it is copied into a new memory 222buffer and the RX descriptor is returned to HW. 223 224Statistics 225========== 226 227The user can obtain ENA device and driver statistics using `ethtool`. 228The driver can collect regular or extended statistics (including 229per-queue stats) from the device. 230 231In addition the driver logs the stats to syslog upon device reset. 232 233MTU 234=== 235 236The driver supports an arbitrarily large MTU with a maximum that is 237negotiated with the device. The driver configures MTU using the 238SetFeature command (ENA_ADMIN_MTU property). The user can change MTU 239via `ip(8)` and similar legacy tools. 240 241Stateless Offloads 242================== 243 244The ENA driver supports: 245 246- IPv4 header checksum offload 247- TCP/UDP over IPv4/IPv6 checksum offloads 248 249RSS 250=== 251 252- The ENA device supports RSS that allows flexible Rx traffic 253 steering. 254- Toeplitz and CRC32 hash functions are supported. 255- Different combinations of L2/L3/L4 fields can be configured as 256 inputs for hash functions. 257- The driver configures RSS settings using the AQ SetFeature command 258 (ENA_ADMIN_RSS_HASH_FUNCTION, ENA_ADMIN_RSS_HASH_INPUT and 259 ENA_ADMIN_RSS_INDIRECTION_TABLE_CONFIG properties). 260- If the NETIF_F_RXHASH flag is set, the 32-bit result of the hash 261 function delivered in the Rx CQ descriptor is set in the received 262 SKB. 263- The user can provide a hash key, hash function, and configure the 264 indirection table through `ethtool(8)`. 265 266DATA PATH 267========= 268 269Tx 270-- 271 272:code:`ena_start_xmit()` is called by the stack. This function does the following: 273 274- Maps data buffers (``skb->data`` and frags). 275- Populates ``ena_buf`` for the push buffer (if the driver and device are 276 in push mode). 277- Prepares ENA bufs for the remaining frags. 278- Allocates a new request ID from the empty ``req_id`` ring. The request 279 ID is the index of the packet in the Tx info. This is used for 280 out-of-order Tx completions. 281- Adds the packet to the proper place in the Tx ring. 282- Calls :code:`ena_com_prepare_tx()`, an ENA communication layer that converts 283 the ``ena_bufs`` to ENA descriptors (and adds meta ENA descriptors as 284 needed). 285 286 * This function also copies the ENA descriptors and the push buffer 287 to the Device memory space (if in push mode). 288 289- Writes a doorbell to the ENA device. 290- When the ENA device finishes sending the packet, a completion 291 interrupt is raised. 292- The interrupt handler schedules NAPI. 293- The :code:`ena_clean_tx_irq()` function is called. This function handles the 294 completion descriptors generated by the ENA, with a single 295 completion descriptor per completed packet. 296 297 * ``req_id`` is retrieved from the completion descriptor. The ``tx_info`` of 298 the packet is retrieved via the ``req_id``. The data buffers are 299 unmapped and ``req_id`` is returned to the empty ``req_id`` ring. 300 * The function stops when the completion descriptors are completed or 301 the budget is reached. 302 303Rx 304-- 305 306- When a packet is received from the ENA device. 307- The interrupt handler schedules NAPI. 308- The :code:`ena_clean_rx_irq()` function is called. This function calls 309 :code:`ena_com_rx_pkt()`, an ENA communication layer function, which returns the 310 number of descriptors used for a new packet, and zero if 311 no new packet is found. 312- :code:`ena_rx_skb()` checks packet length: 313 314 * If the packet is small (len < rx_copybreak), the driver allocates 315 a SKB for the new packet, and copies the packet payload into the 316 SKB data buffer. 317 318 - In this way the original data buffer is not passed to the stack 319 and is reused for future Rx packets. 320 321 * Otherwise the function unmaps the Rx buffer, sets the first 322 descriptor as `skb`'s linear part and the other descriptors as the 323 `skb`'s frags. 324 325- The new SKB is updated with the necessary information (protocol, 326 checksum hw verify result, etc), and then passed to the network 327 stack, using the NAPI interface function :code:`napi_gro_receive()`. 328 329Dynamic RX Buffers (DRB) 330------------------------ 331 332Each RX descriptor in the RX ring is a single memory page (which is either 4KB 333or 16KB long depending on system's configurations). 334To reduce the memory allocations required when dealing with a high rate of small 335packets, the driver tries to reuse the remaining RX descriptor's space if more 336than 2KB of this page remain unused. 337 338A simple example of this mechanism is the following sequence of events: 339 340:: 341 342 1. Driver allocates page-sized RX buffer and passes it to hardware 343 +----------------------+ 344 |4KB RX Buffer | 345 +----------------------+ 346 347 2. A 300Bytes packet is received on this buffer 348 349 3. The driver increases the ref count on this page and returns it back to 350 HW as an RX buffer of size 4KB - 300Bytes = 3796 Bytes 351 +----+--------------------+ 352 |****|3796 Bytes RX Buffer| 353 +----+--------------------+ 354 355This mechanism isn't used when an XDP program is loaded, or when the 356RX packet is less than rx_copybreak bytes (in which case the packet is 357copied out of the RX buffer into the linear part of a new skb allocated 358for it and the RX buffer remains the same size, see `RX copybreak`_). 359