306 lines
12 KiB
Plaintext
306 lines
12 KiB
Plaintext
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Linux kernel driver for Elastic Network Adapter (ENA) family:
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=============================================================
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Overview:
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=========
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ENA is a networking interface designed to make good use of modern CPU
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features and system architectures.
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The ENA device exposes a lightweight management interface with a
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minimal set of memory mapped registers and extendable command set
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through an Admin Queue.
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The driver supports a range of ENA devices, is link-speed independent
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(i.e., the same driver is used for 10GbE, 25GbE, 40GbE, etc.), and has
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a negotiated and extendable feature set.
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Some ENA devices support SR-IOV. This driver is used for both the
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SR-IOV Physical Function (PF) and Virtual Function (VF) devices.
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ENA devices enable high speed and low overhead network traffic
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processing by providing multiple Tx/Rx queue pairs (the maximum number
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is advertised by the device via the Admin Queue), a dedicated MSI-X
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interrupt vector per Tx/Rx queue pair, adaptive interrupt moderation,
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and CPU cacheline optimized data placement.
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The ENA driver supports industry standard TCP/IP offload features such
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as checksum offload and TCP transmit segmentation offload (TSO).
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Receive-side scaling (RSS) is supported for multi-core scaling.
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The ENA driver and its corresponding devices implement health
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monitoring mechanisms such as watchdog, enabling the device and driver
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to recover in a manner transparent to the application, as well as
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debug logs.
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Some of the ENA devices support a working mode called Low-latency
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Queue (LLQ), which saves several more microseconds.
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Supported PCI vendor ID/device IDs:
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===================================
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1d0f:0ec2 - ENA PF
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1d0f:1ec2 - ENA PF with LLQ support
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1d0f:ec20 - ENA VF
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1d0f:ec21 - ENA VF with LLQ support
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ENA Source Code Directory Structure:
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====================================
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ena_com.[ch] - Management communication layer. This layer is
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responsible for the handling all the management
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(admin) communication between the device and the
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driver.
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ena_eth_com.[ch] - Tx/Rx data path.
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ena_admin_defs.h - Definition of ENA management interface.
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ena_eth_io_defs.h - Definition of ENA data path interface.
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ena_common_defs.h - Common definitions for ena_com layer.
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ena_regs_defs.h - Definition of ENA PCI memory-mapped (MMIO) registers.
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ena_netdev.[ch] - Main Linux kernel driver.
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ena_syfsfs.[ch] - Sysfs files.
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ena_ethtool.c - ethtool callbacks.
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ena_pci_id_tbl.h - Supported device IDs.
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Management Interface:
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=====================
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ENA management interface is exposed by means of:
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- PCIe Configuration Space
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- Device Registers
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- Admin Queue (AQ) and Admin Completion Queue (ACQ)
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- Asynchronous Event Notification Queue (AENQ)
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ENA device MMIO Registers are accessed only during driver
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initialization and are not involved in further normal device
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operation.
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AQ is used for submitting management commands, and the
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results/responses are reported asynchronously through ACQ.
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ENA introduces a very small set of management commands with room for
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vendor-specific extensions. Most of the management operations are
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framed in a generic Get/Set feature command.
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The following admin queue commands are supported:
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- Create I/O submission queue
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- Create I/O completion queue
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- Destroy I/O submission queue
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- Destroy I/O completion queue
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- Get feature
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- Set feature
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- Configure AENQ
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- Get statistics
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Refer to ena_admin_defs.h for the list of supported Get/Set Feature
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properties.
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The Asynchronous Event Notification Queue (AENQ) is a uni-directional
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queue used by the ENA device to send to the driver events that cannot
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be reported using ACQ. AENQ events are subdivided into groups. Each
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group may have multiple syndromes, as shown below
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The events are:
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Group Syndrome
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Link state change - X -
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Fatal error - X -
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Notification Suspend traffic
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Notification Resume traffic
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Keep-Alive - X -
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ACQ and AENQ share the same MSI-X vector.
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Keep-Alive is a special mechanism that allows monitoring of the
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device's health. The driver maintains a watchdog (WD) handler which,
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if fired, logs the current state and statistics then resets and
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restarts the ENA device and driver. A Keep-Alive event is delivered by
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the device every second. The driver re-arms the WD upon reception of a
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Keep-Alive event. A missed Keep-Alive event causes the WD handler to
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fire.
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Data Path Interface:
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====================
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I/O operations are based on Tx and Rx Submission Queues (Tx SQ and Rx
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SQ correspondingly). Each SQ has a completion queue (CQ) associated
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with it.
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The SQs and CQs are implemented as descriptor rings in contiguous
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physical memory.
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The ENA driver supports two Queue Operation modes for Tx SQs:
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- Regular mode
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* In this mode the Tx SQs reside in the host's memory. The ENA
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device fetches the ENA Tx descriptors and packet data from host
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memory.
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- Low Latency Queue (LLQ) mode or "push-mode".
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* In this mode the driver pushes the transmit descriptors and the
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first 128 bytes of the packet directly to the ENA device memory
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space. The rest of the packet payload is fetched by the
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device. For this operation mode, the driver uses a dedicated PCI
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device memory BAR, which is mapped with write-combine capability.
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The Rx SQs support only the regular mode.
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Note: Not all ENA devices support LLQ, and this feature is negotiated
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with the device upon initialization. If the ENA device does not
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support LLQ mode, the driver falls back to the regular mode.
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The driver supports multi-queue for both Tx and Rx. This has various
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benefits:
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- Reduced CPU/thread/process contention on a given Ethernet interface.
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- Cache miss rate on completion is reduced, particularly for data
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cache lines that hold the sk_buff structures.
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- Increased process-level parallelism when handling received packets.
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- Increased data cache hit rate, by steering kernel processing of
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packets to the CPU, where the application thread consuming the
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packet is running.
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- In hardware interrupt re-direction.
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Interrupt Modes:
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================
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The driver assigns a single MSI-X vector per queue pair (for both Tx
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and Rx directions). The driver assigns an additional dedicated MSI-X vector
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for management (for ACQ and AENQ).
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Management interrupt registration is performed when the Linux kernel
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probes the adapter, and it is de-registered when the adapter is
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removed. I/O queue interrupt registration is performed when the Linux
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interface of the adapter is opened, and it is de-registered when the
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interface is closed.
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The management interrupt is named:
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ena-mgmnt@pci:<PCI domain:bus:slot.function>
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and for each queue pair, an interrupt is named:
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<interface name>-Tx-Rx-<queue index>
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The ENA device operates in auto-mask and auto-clear interrupt
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modes. That is, once MSI-X is delivered to the host, its Cause bit is
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automatically cleared and the interrupt is masked. The interrupt is
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unmasked by the driver after NAPI processing is complete.
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Interrupt Moderation:
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=====================
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ENA driver and device can operate in conventional or adaptive interrupt
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moderation mode.
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In conventional mode the driver instructs device to postpone interrupt
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posting according to static interrupt delay value. The interrupt delay
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value can be configured through ethtool(8). The following ethtool
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parameters are supported by the driver: tx-usecs, rx-usecs
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In adaptive interrupt moderation mode the interrupt delay value is
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updated by the driver dynamically and adjusted every NAPI cycle
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according to the traffic nature.
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By default ENA driver applies adaptive coalescing on Rx traffic and
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conventional coalescing on Tx traffic.
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Adaptive coalescing can be switched on/off through ethtool(8)
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adaptive_rx on|off parameter.
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The driver chooses interrupt delay value according to the number of
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bytes and packets received between interrupt unmasking and interrupt
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posting. The driver uses interrupt delay table that subdivides the
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range of received bytes/packets into 5 levels and assigns interrupt
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delay value to each level.
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The user can enable/disable adaptive moderation, modify the interrupt
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delay table and restore its default values through sysfs.
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The rx_copybreak is initialized by default to ENA_DEFAULT_RX_COPYBREAK
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and can be configured by the ETHTOOL_STUNABLE command of the
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SIOCETHTOOL ioctl.
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SKB:
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The driver-allocated SKB for frames received from Rx handling using
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NAPI context. The allocation method depends on the size of the packet.
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If the frame length is larger than rx_copybreak, napi_get_frags()
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is used, otherwise netdev_alloc_skb_ip_align() is used, the buffer
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content is copied (by CPU) to the SKB, and the buffer is recycled.
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Statistics:
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===========
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The user can obtain ENA device and driver statistics using ethtool.
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The driver can collect regular or extended statistics (including
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per-queue stats) from the device.
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In addition the driver logs the stats to syslog upon device reset.
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MTU:
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====
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The driver supports an arbitrarily large MTU with a maximum that is
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negotiated with the device. The driver configures MTU using the
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SetFeature command (ENA_ADMIN_MTU property). The user can change MTU
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via ip(8) and similar legacy tools.
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Stateless Offloads:
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===================
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The ENA driver supports:
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- TSO over IPv4/IPv6
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- TSO with ECN
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- IPv4 header checksum offload
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- TCP/UDP over IPv4/IPv6 checksum offloads
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RSS:
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====
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- The ENA device supports RSS that allows flexible Rx traffic
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steering.
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- Toeplitz and CRC32 hash functions are supported.
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- Different combinations of L2/L3/L4 fields can be configured as
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inputs for hash functions.
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- The driver configures RSS settings using the AQ SetFeature command
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(ENA_ADMIN_RSS_HASH_FUNCTION, ENA_ADMIN_RSS_HASH_INPUT and
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ENA_ADMIN_RSS_REDIRECTION_TABLE_CONFIG properties).
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- If the NETIF_F_RXHASH flag is set, the 32-bit result of the hash
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function delivered in the Rx CQ descriptor is set in the received
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SKB.
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- The user can provide a hash key, hash function, and configure the
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indirection table through ethtool(8).
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DATA PATH:
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==========
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Tx:
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---
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end_start_xmit() is called by the stack. This function does the following:
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- Maps data buffers (skb->data and frags).
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- Populates ena_buf for the push buffer (if the driver and device are
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in push mode.)
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- Prepares ENA bufs for the remaining frags.
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- Allocates a new request ID from the empty req_id ring. The request
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ID is the index of the packet in the Tx info. This is used for
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out-of-order TX completions.
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- Adds the packet to the proper place in the Tx ring.
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- Calls ena_com_prepare_tx(), an ENA communication layer that converts
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the ena_bufs to ENA descriptors (and adds meta ENA descriptors as
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needed.)
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* This function also copies the ENA descriptors and the push buffer
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to the Device memory space (if in push mode.)
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- Writes doorbell to the ENA device.
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- When the ENA device finishes sending the packet, a completion
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interrupt is raised.
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- The interrupt handler schedules NAPI.
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- The ena_clean_tx_irq() function is called. This function handles the
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completion descriptors generated by the ENA, with a single
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completion descriptor per completed packet.
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* req_id is retrieved from the completion descriptor. The tx_info of
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the packet is retrieved via the req_id. The data buffers are
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unmapped and req_id is returned to the empty req_id ring.
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* The function stops when the completion descriptors are completed or
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the budget is reached.
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Rx:
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---
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- When a packet is received from the ENA device.
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- The interrupt handler schedules NAPI.
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- The ena_clean_rx_irq() function is called. This function calls
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ena_rx_pkt(), an ENA communication layer function, which returns the
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number of descriptors used for a new unhandled packet, and zero if
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no new packet is found.
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- Then it calls the ena_clean_rx_irq() function.
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- ena_eth_rx_skb() checks packet length:
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* If the packet is small (len < rx_copybreak), the driver allocates
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a SKB for the new packet, and copies the packet payload into the
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SKB data buffer.
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- In this way the original data buffer is not passed to the stack
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and is reused for future Rx packets.
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* Otherwise the function unmaps the Rx buffer, then allocates the
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new SKB structure and hooks the Rx buffer to the SKB frags.
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- The new SKB is updated with the necessary information (protocol,
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checksum hw verify result, etc.), and then passed to the network
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stack, using the NAPI interface function napi_gro_receive().
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