Based on kernel version 4.15. Page generated on 2018-01-29 10:00 EST.
1 Linux kernel driver for Elastic Network Adapter (ENA) family: 2 ============================================================= 3 4 Overview: 5 ========= 6 ENA is a networking interface designed to make good use of modern CPU 7 features and system architectures. 8 9 The ENA device exposes a lightweight management interface with a 10 minimal set of memory mapped registers and extendable command set 11 through an Admin Queue. 12 13 The driver supports a range of ENA devices, is link-speed independent 14 (i.e., the same driver is used for 10GbE, 25GbE, 40GbE, etc.), and has 15 a negotiated and extendable feature set. 16 17 Some ENA devices support SR-IOV. This driver is used for both the 18 SR-IOV Physical Function (PF) and Virtual Function (VF) devices. 19 20 ENA devices enable high speed and low overhead network traffic 21 processing by providing multiple Tx/Rx queue pairs (the maximum number 22 is advertised by the device via the Admin Queue), a dedicated MSI-X 23 interrupt vector per Tx/Rx queue pair, adaptive interrupt moderation, 24 and CPU cacheline optimized data placement. 25 26 The ENA driver supports industry standard TCP/IP offload features such 27 as checksum offload and TCP transmit segmentation offload (TSO). 28 Receive-side scaling (RSS) is supported for multi-core scaling. 29 30 The ENA driver and its corresponding devices implement health 31 monitoring mechanisms such as watchdog, enabling the device and driver 32 to recover in a manner transparent to the application, as well as 33 debug logs. 34 35 Some of the ENA devices support a working mode called Low-latency 36 Queue (LLQ), which saves several more microseconds. 37 38 Supported PCI vendor ID/device IDs: 39 =================================== 40 1d0f:0ec2 - ENA PF 41 1d0f:1ec2 - ENA PF with LLQ support 42 1d0f:ec20 - ENA VF 43 1d0f:ec21 - ENA VF with LLQ support 44 45 ENA Source Code Directory Structure: 46 ==================================== 47 ena_com.[ch] - Management communication layer. This layer is 48 responsible for the handling all the management 49 (admin) communication between the device and the 50 driver. 51 ena_eth_com.[ch] - Tx/Rx data path. 52 ena_admin_defs.h - Definition of ENA management interface. 53 ena_eth_io_defs.h - Definition of ENA data path interface. 54 ena_common_defs.h - Common definitions for ena_com layer. 55 ena_regs_defs.h - Definition of ENA PCI memory-mapped (MMIO) registers. 56 ena_netdev.[ch] - Main Linux kernel driver. 57 ena_syfsfs.[ch] - Sysfs files. 58 ena_ethtool.c - ethtool callbacks. 59 ena_pci_id_tbl.h - Supported device IDs. 60 61 Management Interface: 62 ===================== 63 ENA management interface is exposed by means of: 64 - PCIe Configuration Space 65 - Device Registers 66 - Admin Queue (AQ) and Admin Completion Queue (ACQ) 67 - Asynchronous Event Notification Queue (AENQ) 68 69 ENA device MMIO Registers are accessed only during driver 70 initialization and are not involved in further normal device 71 operation. 72 73 AQ is used for submitting management commands, and the 74 results/responses are reported asynchronously through ACQ. 75 76 ENA introduces a very small set of management commands with room for 77 vendor-specific extensions. Most of the management operations are 78 framed in a generic Get/Set feature command. 79 80 The following admin queue commands are supported: 81 - Create I/O submission queue 82 - Create I/O completion queue 83 - Destroy I/O submission queue 84 - Destroy I/O completion queue 85 - Get feature 86 - Set feature 87 - Configure AENQ 88 - Get statistics 89 90 Refer to ena_admin_defs.h for the list of supported Get/Set Feature 91 properties. 92 93 The Asynchronous Event Notification Queue (AENQ) is a uni-directional 94 queue used by the ENA device to send to the driver events that cannot 95 be reported using ACQ. AENQ events are subdivided into groups. Each 96 group may have multiple syndromes, as shown below 97 98 The events are: 99 Group Syndrome 100 Link state change - X - 101 Fatal error - X - 102 Notification Suspend traffic 103 Notification Resume traffic 104 Keep-Alive - X - 105 106 ACQ and AENQ share the same MSI-X vector. 107 108 Keep-Alive is a special mechanism that allows monitoring of the 109 device's health. The driver maintains a watchdog (WD) handler which, 110 if fired, logs the current state and statistics then resets and 111 restarts the ENA device and driver. A Keep-Alive event is delivered by 112 the device every second. The driver re-arms the WD upon reception of a 113 Keep-Alive event. A missed Keep-Alive event causes the WD handler to 114 fire. 115 116 Data Path Interface: 117 ==================== 118 I/O operations are based on Tx and Rx Submission Queues (Tx SQ and Rx 119 SQ correspondingly). Each SQ has a completion queue (CQ) associated 120 with it. 121 122 The SQs and CQs are implemented as descriptor rings in contiguous 123 physical memory. 124 125 The ENA driver supports two Queue Operation modes for Tx SQs: 126 - Regular mode 127 * In this mode the Tx SQs reside in the host's memory. The ENA 128 device fetches the ENA Tx descriptors and packet data from host 129 memory. 130 - Low Latency Queue (LLQ) mode or "push-mode". 131 * In this mode the driver pushes the transmit descriptors and the 132 first 128 bytes of the packet directly to the ENA device memory 133 space. The rest of the packet payload is fetched by the 134 device. For this operation mode, the driver uses a dedicated PCI 135 device memory BAR, which is mapped with write-combine capability. 136 137 The Rx SQs support only the regular mode. 138 139 Note: Not all ENA devices support LLQ, and this feature is negotiated 140 with the device upon initialization. If the ENA device does not 141 support LLQ mode, the driver falls back to the regular mode. 142 143 The driver supports multi-queue for both Tx and Rx. This has various 144 benefits: 145 - Reduced CPU/thread/process contention on a given Ethernet interface. 146 - Cache miss rate on completion is reduced, particularly for data 147 cache lines that hold the sk_buff structures. 148 - Increased process-level parallelism when handling received packets. 149 - Increased data cache hit rate, by steering kernel processing of 150 packets to the CPU, where the application thread consuming the 151 packet is running. 152 - In hardware interrupt re-direction. 153 154 Interrupt Modes: 155 ================ 156 The driver assigns a single MSI-X vector per queue pair (for both Tx 157 and Rx directions). The driver assigns an additional dedicated MSI-X vector 158 for management (for ACQ and AENQ). 159 160 Management interrupt registration is performed when the Linux kernel 161 probes the adapter, and it is de-registered when the adapter is 162 removed. I/O queue interrupt registration is performed when the Linux 163 interface of the adapter is opened, and it is de-registered when the 164 interface is closed. 165 166 The management interrupt is named: 167 ena-mgmnt@pci:<PCI domain:bus:slot.function> 168 and for each queue pair, an interrupt is named: 169 <interface name>-Tx-Rx-<queue index> 170 171 The ENA device operates in auto-mask and auto-clear interrupt 172 modes. That is, once MSI-X is delivered to the host, its Cause bit is 173 automatically cleared and the interrupt is masked. The interrupt is 174 unmasked by the driver after NAPI processing is complete. 175 176 Interrupt Moderation: 177 ===================== 178 ENA driver and device can operate in conventional or adaptive interrupt 179 moderation mode. 180 181 In conventional mode the driver instructs device to postpone interrupt 182 posting according to static interrupt delay value. The interrupt delay 183 value can be configured through ethtool(8). The following ethtool 184 parameters are supported by the driver: tx-usecs, rx-usecs 185 186 In adaptive interrupt moderation mode the interrupt delay value is 187 updated by the driver dynamically and adjusted every NAPI cycle 188 according to the traffic nature. 189 190 By default ENA driver applies adaptive coalescing on Rx traffic and 191 conventional coalescing on Tx traffic. 192 193 Adaptive coalescing can be switched on/off through ethtool(8) 194 adaptive_rx on|off parameter. 195 196 The driver chooses interrupt delay value according to the number of 197 bytes and packets received between interrupt unmasking and interrupt 198 posting. The driver uses interrupt delay table that subdivides the 199 range of received bytes/packets into 5 levels and assigns interrupt 200 delay value to each level. 201 202 The user can enable/disable adaptive moderation, modify the interrupt 203 delay table and restore its default values through sysfs. 204 205 The rx_copybreak is initialized by default to ENA_DEFAULT_RX_COPYBREAK 206 and can be configured by the ETHTOOL_STUNABLE command of the 207 SIOCETHTOOL ioctl. 208 209 SKB: 210 The driver-allocated SKB for frames received from Rx handling using 211 NAPI context. The allocation method depends on the size of the packet. 212 If the frame length is larger than rx_copybreak, napi_get_frags() 213 is used, otherwise netdev_alloc_skb_ip_align() is used, the buffer 214 content is copied (by CPU) to the SKB, and the buffer is recycled. 215 216 Statistics: 217 =========== 218 The user can obtain ENA device and driver statistics using ethtool. 219 The driver can collect regular or extended statistics (including 220 per-queue stats) from the device. 221 222 In addition the driver logs the stats to syslog upon device reset. 223 224 MTU: 225 ==== 226 The driver supports an arbitrarily large MTU with a maximum that is 227 negotiated with the device. The driver configures MTU using the 228 SetFeature command (ENA_ADMIN_MTU property). The user can change MTU 229 via ip(8) and similar legacy tools. 230 231 Stateless Offloads: 232 =================== 233 The ENA driver supports: 234 - TSO over IPv4/IPv6 235 - TSO with ECN 236 - IPv4 header checksum offload 237 - TCP/UDP over IPv4/IPv6 checksum offloads 238 239 RSS: 240 ==== 241 - The ENA device supports RSS that allows flexible Rx traffic 242 steering. 243 - Toeplitz and CRC32 hash functions are supported. 244 - Different combinations of L2/L3/L4 fields can be configured as 245 inputs for hash functions. 246 - The driver configures RSS settings using the AQ SetFeature command 247 (ENA_ADMIN_RSS_HASH_FUNCTION, ENA_ADMIN_RSS_HASH_INPUT and 248 ENA_ADMIN_RSS_REDIRECTION_TABLE_CONFIG properties). 249 - If the NETIF_F_RXHASH flag is set, the 32-bit result of the hash 250 function delivered in the Rx CQ descriptor is set in the received 251 SKB. 252 - The user can provide a hash key, hash function, and configure the 253 indirection table through ethtool(8). 254 255 DATA PATH: 256 ========== 257 Tx: 258 --- 259 end_start_xmit() is called by the stack. This function does the following: 260 - Maps data buffers (skb->data and frags). 261 - Populates ena_buf for the push buffer (if the driver and device are 262 in push mode.) 263 - Prepares ENA bufs for the remaining frags. 264 - Allocates a new request ID from the empty req_id ring. The request 265 ID is the index of the packet in the Tx info. This is used for 266 out-of-order TX completions. 267 - Adds the packet to the proper place in the Tx ring. 268 - Calls ena_com_prepare_tx(), an ENA communication layer that converts 269 the ena_bufs to ENA descriptors (and adds meta ENA descriptors as 270 needed.) 271 * This function also copies the ENA descriptors and the push buffer 272 to the Device memory space (if in push mode.) 273 - Writes doorbell to the ENA device. 274 - When the ENA device finishes sending the packet, a completion 275 interrupt is raised. 276 - The interrupt handler schedules NAPI. 277 - The ena_clean_tx_irq() function is called. This function handles the 278 completion descriptors generated by the ENA, with a single 279 completion descriptor per completed packet. 280 * req_id is retrieved from the completion descriptor. The tx_info of 281 the packet is retrieved via the req_id. The data buffers are 282 unmapped and req_id is returned to the empty req_id ring. 283 * The function stops when the completion descriptors are completed or 284 the budget is reached. 285 286 Rx: 287 --- 288 - When a packet is received from the ENA device. 289 - The interrupt handler schedules NAPI. 290 - The ena_clean_rx_irq() function is called. This function calls 291 ena_rx_pkt(), an ENA communication layer function, which returns the 292 number of descriptors used for a new unhandled packet, and zero if 293 no new packet is found. 294 - Then it calls the ena_clean_rx_irq() function. 295 - ena_eth_rx_skb() checks packet length: 296 * If the packet is small (len < rx_copybreak), the driver allocates 297 a SKB for the new packet, and copies the packet payload into the 298 SKB data buffer. 299 - In this way the original data buffer is not passed to the stack 300 and is reused for future Rx packets. 301 * Otherwise the function unmaps the Rx buffer, then allocates the 302 new SKB structure and hooks the Rx buffer to the SKB frags. 303 - The new SKB is updated with the necessary information (protocol, 304 checksum hw verify result, etc.), and then passed to the network 305 stack, using the NAPI interface function napi_gro_receive().