ena

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.

Driver compilation

Prerequisites:

Amazon Linux

sudo yum update
sudo reboot
sudo yum install kernel-devel-$(uname -r) git

RHEL

sudo yum update
sudo reboot
sudo yum install gcc kernel-devel-$(uname -r) git

Ubuntu

sudo apt-get update
sudo apt install linux-headers-$(uname -r)
sudo apt-get install make gcc

SLES

sudo zypper update
sudo reboot
sudo zypper install make gcc kernel-default-devel git

CentOS

sudo yum update
sudo reboot
sudo yum install kernel-devel-$(uname -r) git

Compilation:

Run

make [UBUNTU_ABI=<ABI>]

in the kernel/linux/ena/ folder. ena.ko is created inside the folder

Optional compilation parameters:

For most kernel versions no special compilation parameters are needed. The exceptions are:

• UBUNTU_ABI=<ABI>

  For Ubuntu kernel versions < 3.13.0-31 add the special parameter UBUNTU_ABI=<ABI>. The ABI of an ubuntu kernel is the 4th integer in the kernel version string. To see the kernel version you can run uname -r.

  Example: if uname -r yields the output 3.13.0-29-generic, then the ABI is 29, and the compilation command is make UBUNTU_ABI=29.

Loading driver:

If the driver was compiled using ENA_PHC_INCLUDE environment variable set then ptp module might need to be loaded prior to loading the driver (see PHC for more info).

ENA driver can be (re)loaded using:

sudo rmmod ena && sudo insmod ena.ko

Please note, the following messages might appear during OS boot:

ena: loading out-of-tree module taints kernel.
ena: module verification failed: signature and/or required key missing - tainting kernel

These messages are informational and indicate that out-of-tree driver is being used, and do not affect driver operation.

Driver installation using dkms

Please refer to supported instances for the list of instance types supporting ENA.

Please also make sure Enhanced Networking is enabled on your instance as specified in test-enhanced-networking-ena.

Installing dkms:

Amazon Linux

sudo yum install dkms

RHEL

# Replace the 7 with the RHEL version you have (For example - for RHEL 9 use 9)
sudo yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
sudo yum install dkms

Ubuntu

sudo apt-get install dkms

SLES 15

# Replace the 15.4 with the correct SLES 15 version
# For arm instances replace x86_64 with aarch64
sudo SUSEConnect --product PackageHub/15.4/x86_64
sudo zypper refresh
sudo zypper install dkms

CentOS

sudo yum install --enablerepo=extras epel-release
sudo yum install dkms

Installation from Source Code

git clone https://github.com/dell/dkms.git
cd dkms
sudo make install

Installing Driver with dkms:

git clone https://github.com/amzn/amzn-drivers.git
sudo mv amzn-drivers /usr/src/amzn-drivers-X.Y.Z (X.Y.Z = driver version)
sudo vi /usr/src/amzn-drivers-X.Y.Z/dkms.conf

# Paste the following and save the file:

PACKAGE_NAME="ena"
PACKAGE_VERSION="1.0.0"
CLEAN="make -C kernel/linux/ena clean"
MAKE="make -C kernel/linux/ena/ BUILD_KERNEL=${kernelver}"
BUILT_MODULE_NAME[0]="ena"
BUILT_MODULE_LOCATION="kernel/linux/ena"
DEST_MODULE_LOCATION[0]="/updates"
DEST_MODULE_NAME[0]="ena"
REMAKE_INITRD="yes"
AUTOINSTALL="yes"

sudo dkms add -m amzn-drivers -v X.Y.Z
sudo dkms build -m amzn-drivers -v X.Y.Z
sudo dkms install -m amzn-drivers -v X.Y.Z

# NOTE - To have the driver installation retain across reboots
# please go over the next section as well.

Making Sure the Installed Driver is Loaded During Boot:

In some cases it is necessary to perform some extra steps to make sure that the installed driver loads during boot.

RHEL and SLES 12

sudo dracut -f -v

SLES

sudo vi /etc/modprobe.d/10-unsupported-modules.conf

# Paste the following into file and save it:

allow_unsupported_modules 1

Loading the Newly Installed Driver:

You can either reboot the instance:

sudo reboot

Or simply reload the driver:

sudo modprobe -r ena; sudo modprobe ena

Module Parameters

rx_queue_size:	Controls the number of requested entries in the Rx Queue. Increasing the Rx queue size can be useful in situations where rx drops are observed in loaded systems with NAPI not scheduled fast enough. The value provided will be rounded down to a power of 2. Default value 1024. Max value is up to 16K (16384), depending on the instance type, and the actual value can be seen by running ethtool -g. The Min value is 256. The actual number of entries in the queues is negotiated with the device.
force_large_llq_header:	Controls the maximum supported packet header size when LLQ is enabled. When this parameter is set to 0 (default value), the maximum packet header size is set to 96 bytes. When this parameter is set to a non 0 value, the maximum packet header size is set to 224 bytes, and the Tx queue size is reduced by half.
num_io_queues:	Controls the number of requested IO queues. The maximum possible number is set to be MIN(maximum IO queues supported by the device, number of MSI-X vectors supported by the device, number of online CPUs). The minimum number of queues is 1. If the number of queues given is outside of the range, the number of queues will be set to the closest number from within the range.
lpc_size:	Controls the size of the Local Page Cache size which would be lpc_size * 1024. Maximum value for this parameter is 32, and a value of 0 disables it completely. The default value is 2. See LPC section in this README for a description of this system.
phc_enable:	Controls the enablement of the PHC feature. The default value is 0 (Disabled). Notice that PHC must be supported by the kernel and the device.

Disable Predictable Network Names:

When predictable network naming is enabled, Linux might change the device name and affect the network configuration. This can lead to a loss of network on boot. To disable this feature add net.ifnames=0 to the kernel boot params.

Edit /etc/default/grub and add net.ifnames=0 to GRUB_CMDLINE_LINUX. On Ubuntu run update-grub as well

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_sysfs.[ch]	Sysfs files.
ena_lpc.[ch]	Local Page Cache files (see LPC for more info)
ena_ethtool.c	ethtool callbacks.
ena_xdp.[ch]	XDP files
ena_pci_id_tbl.h	Supported device IDs.
ena_phc.[ch]	PTP hardware clock infrastructure (see PHC for more info)

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 few bytes of the packet (negotiable parameter) 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) using adaptive_rx on|off parameter.

More information about Adaptive Interrupt Moderation (DIM) can be found in https://elixir.bootlin.com/linux/latest/source/Documentation/networking/net_dim.rst

RX copybreak

The rx_copybreak is initialized by default to ENA_DEFAULT_RX_COPYBREAK and can be configured using ethtool --set-tunable. This option is supported for kernel versions 3.18 and newer. Alternatively copybreak values can be configured by the sysfs path /sys/bus/pci/devices/<domain:bus:slot.function>/rx_copybreak.

This option controls the maximum packet length for which the RX descriptor it was received on would be recycled. When a packet smaller than RX copybreak bytes is received, it is copied into a new memory buffer and the RX descriptor is returned to HW.

Local Page Cache (LPC)

ENA Linux driver allows to reduce lock contention and improve CPU usage by allocating Rx buffers from a page cache rather than from Linux memory system (PCP or buddy allocator). The cache is created and binded per Rx queue, and pages allocated for the queue are stored in the cache (up to cache maximum size).

To set the cache size, one can specify lpc_size module parameter, which would create a cache that can hold up to lpc_size * 1024 pages for each Rx queue. Setting it to 0, would disable this feature completely (fallback to regular page allocations).

The feature can be toggled between on/off state using ethtool private flags, e.g.

ethtool --set-priv-flags eth1 local_page_cache off

The cache usage for each queue can be monitored using ethtool -S counters. Where:

• rx_queue#_lpc_warm_up - number of pages that were allocated and stored in the cache
• rx_queue#_lpc_full - number of pages that were allocated without using the cache because it didn't have free pages
• rx_queue#_lpc_wrong_numa - number of pages from the cache that belong to a different NUMA node than the CPU which runs the NAPI routine. In this case, the driver would try to allocate a new page from the same NUMA node instead

Note that lpc_size is set to 2 by default and cannot exceed 32. Also LPC is disabled when using XDP or when using less than 16 queue pairs. Increasing the cache size might result in higher memory usage, and should be handled with care.

Large Low-Latency Queue (Large LLQ)

The standard LLQ entry size of 128 bytes allows for a maximum of 96 bytes of packet header size which sometimes is not enough (e.g. when using tunneling). Enabling large LLQ by increasing LLQ entry size to 256 bytes, allows a maximum header size of 224 bytes. This comes with the penalty of reducing the number of LLQ entries in the TX queue by 2 (i.e. from 1024 to 512).

This feature is supported from EC2 4th generation instance-types.

Note: Starting from 2.9.0g release, large LLQ is enabled by default on all EC2 6th generation instance-types and on. Due to HW limitations, enabling large LLQ implies that the TX queue size is reduced to 512. Starting from EC2 7th generation instance-types, the Tx queue size may be increased back to 1024 while large LLQ is enabled by invoking the relevant ethtool commands.

Large LLQ configuration

Large LLQ can be configured in several ways:

• module parameter:

sudo insmod ena.ko force_large_llq_header=1

• ethtool:

ethtool -G [interface] tx-push-buf-len [number of bytes: 96 or 224]

Note that the minimal requirements for using ethtool are Linux Kernel v6.4 and above and ethtool v6.4 and above.

• sysfs:

echo 1 | sudo tee /sys/bus/pci/devices/<domain:bus:slot.function>/large_llq_header
# for example:
echo 1 | sudo tee /sys/bus/pci/devices/0000:00:06.0/large_llq_header

After changing LLQ configuration, a log will be printed to indicate whether Large LLQ is enabled or disabled:

ENA Large is LLQ enabled/disabled

Large LLQ query

Large LLQ can be queried in several ways:

• ethtool:

Check the value of TX push buff len under Current hardware settings

ethtool -g [interface]

• sysfs:

cat /sys/bus/pci/devices/<domain:bus:slot.function>/large_llq_header
# for example:
cat /sys/bus/pci/devices/0000:00:06.0/large_llq_header

PTP Hardware Clock (PHC)

ENA Linux driver supports PTP hardware clock providing timestamp reference to achieve nanosecond accuracy.

PHC support

PHC depends on the PTP module, which needs to be either loaded as a module or compiled into the kernel.

Verify if the PTP module is present:

grep -w '^CONFIG_PTP_1588_CLOCK=[ym]' /boot/config-`uname -r`

• If no output is provided, the ENA driver cannot be loaded with PHC support.
• CONFIG_PTP_1588_CLOCK=y: the PTP module is already compiled and loaded inside the kernel binary file.
• CONFIG_PTP_1588_CLOCK=m: the PTP module needs to be loaded prior to loading the ENA driver:

Load PTP module:

sudo modprobe ptp

PHC compilation

This feature is enabled only with the ENA_PHC_INCLUDE environment variable set when compiling the driver:

ENA_PHC_INCLUDE=1 make

PHC activation

The feature is turned off by default, in order to turn the feature on, the ENA driver can be loaded in the following way:

• module parameter:

sudo insmod ena.ko phc_enable=1

All available PTP clock sources can be tracked here:

ls /sys/class/ptp

PHC support and capabilities can be verified using ethtool:

ethtool -T <interface>

PHC timestamp

To retrieve PHC timestamp, use ptp-userspace-api, usage example using testptp:

testptp -d /dev/ptp$(ethtool -T <interface> | awk '/PTP Hardware Clock:/ {print $NF}') -k 1

PHC get time requests should be within reasonable bounds, avoid excessive utilization to ensure optimal performance and efficiency. The ENA device restricts the frequency of PHC get time requests to a maximum of 125 requests per second. If this limit is surpassed, the get time request will fail, leading to an increment in the phc_err statistic.

PHC error bound

PTP HW clock error bound refers to the maximum allowable difference between the clock of the device and the reference clock. The error bound is used to ensure that the clock of the device remains within a certain level of accuracy relative to the reference clock. The error bound (expressed in nanoseconds) is calculated by the device and is retrieved and cached by the driver upon every get PHC timestamp request.

To retrieve the cached PHC error bound value, use the following:

sysfs:

cat /sys/bus/pci/devices/<domain:bus:slot.function>/phc_error_bound

PHC statistics

PHC can be monitored using ethtool -S counters:

phc_cnt	Number of successful retrieved timestamps (below expire timeout).
phc_exp	Number of expired retrieved timestamps (above expire timeout).
phc_skp	Number of skipped get time attempts (during block period).
phc_err	Number of failed get time attempts due to timestamp/error bound errors (entering into block state). Must remain below 1% of all PHC requests to maintain the desired level of accuracy and reliability.

PHC timeouts:

expire	Max time for a valid timestamp retrieval, passing this threshold will fail the get time request and block new requests until block timeout.
block	Blocking period starts once get time request expires or fails, all get time requests during block period will be skipped.

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.

On supported instance types, the statistics will also include the ENA Express data (fields prefixed with ena_srd). For a complete documentation of ENA Express data refer to ena-express-monitor

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.
• The input to the RSS hash function is {Hash key, Source IP, Destination IP, Source Port, Destination Port} (ports are used only for packets that have a Transport layer).
• The input to the RSS hash function is not configurable (other than changing the hash key for drivers and devices that support it).
• 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

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 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 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 ena_clean_rx_irq() function is called. This function calls 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.
• ena_rx_skb() checks packet length:
  • If the packet is small (len < rx_copybreak), the driver allocates an skb for the new packet, and copies the packet's payload into the skb's linear part.
    • 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 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
   +----------------------------+
   | 4096 Bytes 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 the
   HW as an RX buffer of size 3796 Bytes (4096 - 300)
   +-----+----------------------+
   |*****| 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).

Flow Steering (ntuple)

The ENA Linux driver supports Rx flow steering using ntuple filters, which allow packets to be directed to specific queues based on defined flow criteria. This feature enables efficient packet processing by steering specific traffic flows to dedicated CPU cores, improving overall system performance.

The resources used to configure the flow steering rules are allocated as a contiguous table in memory. The number of entries available in this table is determined by the number of virtual CPUs on the host. The table is shared across all interfaces on the host, meaning that an entry used by one interface cannot be used by another interface until the rule is removed.

To verify that the feature is supported, run ethtool -k and expect the output ntuple-filters: on. The feature is supported starting from EC2 7th generation instance-types.

Usage example

Adding a flow steering rule:

(configuring IPv4 tcp traffic with destination port 5001 to queue idx 1, set this rule to index 6 in flow steering table)

ethtool -N eth1 flow-type tcp4 dst-port 5001 action 1 loc 6

Deleting a flow steering rule:

ethtool -N eth1 delete 6

Viewing the list of configured rules:

ethtool -n eth1

# ethtool -n eth1
8 RX rings available
Total 1 rules

Filter: 6
Rule Type: TCP over IPv4
Src IP addr: 0.0.0.0 mask: 255.255.255.255
Dest IP addr: 0.0.0.0 mask: 255.255.255.255
TOS: 0x0 mask: 0xff
Src port: 0 mask: 0xffff
Dest port: 5001 mask: 0x0
Action: Direct to queue 1

AF XDP Native Support (zero copy)

ENA driver supports native AF XDP (zero copy). To make a channel (TX/RX queue pair) zero copy, its index should meet the following criteria:

• It has to be within the bounds of the configured channels.
• It has to be smaller than half of the maximum channel number. E.g. if an instance supports a maximum of 32 channels, zero-copy channels can be configured on channels 0 through 15.

Both the currently configured channels and the maximum available for the instance can be queried using ethtool -l.