5. MLX4 poll mode driver library

The MLX4 poll mode driver library (librte_pmd_mlx4) implements support for Mellanox ConnectX-3 EN 10/40 Gbps adapters as well as their virtual functions (VF) in SR-IOV context.

Information and documentation about this family of adapters can be found on the Mellanox website. Help is also provided by the Mellanox community.

There is also a section dedicated to this poll mode driver.


Due to external dependencies, this driver is disabled by default. It must be enabled manually by setting CONFIG_RTE_LIBRTE_MLX4_PMD=y and recompiling DPDK.


CONFIG_RTE_BUILD_COMBINE_LIBS with CONFIG_RTE_BUILD_SHARED_LIB is not supported and thus the compilation will fail with this configuration.

5.1. Implementation details

Most Mellanox ConnectX-3 devices provide two ports but expose a single PCI bus address, thus unlike most drivers, librte_pmd_mlx4 registers itself as a PCI driver that allocates one Ethernet device per detected port.

For this reason, one cannot white/blacklist a single port without also white/blacklisting the others on the same device.

Besides its dependency on libibverbs (that implies libmlx4 and associated kernel support), librte_pmd_mlx4 relies heavily on system calls for control operations such as querying/updating the MTU and flow control parameters.

For security reasons and robustness, this driver only deals with virtual memory addresses. The way resources allocations are handled by the kernel combined with hardware specifications that allow it to handle virtual memory addresses directly ensure that DPDK applications cannot access random physical memory (or memory that does not belong to the current process).

This capability allows the PMD to coexist with kernel network interfaces which remain functional, although they stop receiving unicast packets as long as they share the same MAC address.

Compiling librte_pmd_mlx4 causes DPDK to be linked against libibverbs.

5.2. Features and limitations

  • RSS, also known as RCA, is supported. In this mode the number of configured RX queues must be a power of two.
  • VLAN filtering is supported.
  • Link state information is provided.
  • Promiscuous mode is supported.
  • All multicast mode is supported.
  • Multiple MAC addresses (unicast, multicast) can be configured.
  • Scattered packets are supported for TX and RX.
  • Inner L3/L4 (IP, TCP and UDP) TX/RX checksum offloading and validation.
  • Outer L3 (IP) TX/RX checksum offloading and validation for VXLAN frames.
  • RSS hash key cannot be modified.
  • Hardware counters are not implemented (they are software counters).

5.3. Configuration

5.3.1. Compilation options

These options can be modified in the .config file.

  • CONFIG_RTE_LIBRTE_MLX4_PMD (default n)

    Toggle compilation of librte_pmd_mlx4 itself.


    Toggle debugging code and stricter compilation flags. Enabling this option adds additional run-time checks and debugging messages at the cost of lower performance.


    Number of scatter/gather elements (SGEs) per work request (WR). Lowering this number improves performance but also limits the ability to receive scattered packets (packets that do not fit a single mbuf). The default value is a safe tradeoff.


    Amount of data to be inlined during TX operations. Improves latency but lowers throughput.


    Maximum number of cached memory pools (MPs) per TX queue. Each MP from which buffers are to be transmitted must be associated to memory regions (MRs). This is a slow operation that must be cached.

    This value is always 1 for RX queues since they use a single MP.


    Toggle software counters. No counters are available if this option is disabled since hardware counters are not supported.

5.3.2. Environment variables


    A nonzero value enables inline receive for packets up to that size. May significantly improve performance in some cases but lower it in others. Requires careful testing.

5.3.3. Run-time configuration

  • The only constraint when RSS mode is requested is to make sure the number of RX queues is a power of two. This is a hardware requirement.
  • librte_pmd_mlx4 brings kernel network interfaces up during initialization because it is affected by their state. Forcing them down prevents packets reception.
  • ethtool operations on related kernel interfaces also affect the PMD.

5.3.4. Kernel module parameters

The mlx4_core kernel module has several parameters that affect the behavior and/or the performance of librte_pmd_mlx4. Some of them are described below.

  • num_vfs (integer or triplet, optionally prefixed by device address strings)

    Create the given number of VFs on the specified devices.

  • log_num_mgm_entry_size (integer)

    Device-managed flow steering (DMFS) is required by DPDK applications. It is enabled by using a negative value, the last four bits of which have a special meaning.

    • -1: force device-managed flow steering (DMFS).
    • -7: configure optimized steering mode to improve performance with the following limitation: VLAN filtering is not supported with this mode. This is the recommended mode in case VLAN filter is not needed.

5.4. Prerequisites

This driver relies on external libraries and kernel drivers for resources allocations and initialization. The following dependencies are not part of DPDK and must be installed separately:

  • libibverbs

    User space verbs framework used by librte_pmd_mlx4. This library provides a generic interface between the kernel and low-level user space drivers such as libmlx4.

    It allows slow and privileged operations (context initialization, hardware resources allocations) to be managed by the kernel and fast operations to never leave user space.

  • libmlx4

    Low-level user space driver library for Mellanox ConnectX-3 devices, it is automatically loaded by libibverbs.

    This library basically implements send/receive calls to the hardware queues.

  • Kernel modules (mlnx-ofed-kernel)

    They provide the kernel-side verbs API and low level device drivers that manage actual hardware initialization and resources sharing with user space processes.

    Unlike most other PMDs, these modules must remain loaded and bound to their devices:

    • mlx4_core: hardware driver managing Mellanox ConnectX-3 devices.
    • mlx4_en: Ethernet device driver that provides kernel network interfaces.
    • mlx4_ib: InifiniBand device driver.
    • ib_uverbs: user space driver for verbs (entry point for libibverbs).
  • Firmware update

    Mellanox OFED releases include firmware updates for ConnectX-3 adapters.

    Because each release provides new features, these updates must be applied to match the kernel modules and libraries they come with.


Both libraries are BSD and GPL licensed. Linux kernel modules are GPL licensed.

Currently supported by DPDK:

  • Mellanox OFED 3.0.
  • Firmware version 2.34.5000 and higher.

5.4.1. Getting Mellanox OFED

While these libraries and kernel modules are available on OpenFabrics Alliance’s website and provided by package managers on most distributions, this PMD requires Ethernet extensions that may not be supported at the moment (this is a work in progress).

Mellanox OFED includes the necessary support and should be used in the meantime. For DPDK, only libibverbs, libmlx4, mlnx-ofed-kernel packages and firmware updates are required from that distribution.


Several versions of Mellanox OFED are available. Installing the version this DPDK release was developed and tested against is strongly recommended. Please check the prerequisites.

5.5. Usage example

This section demonstrates how to launch testpmd with Mellanox ConnectX-3 devices managed by librte_pmd_mlx4.

  1. Load the kernel modules:

    modprobe -a ib_uverbs mlx4_en mlx4_core mlx4_ib


    User space I/O kernel modules (uio and igb_uio) are not used and do not have to be loaded.

  2. Make sure Ethernet interfaces are in working order and linked to kernel verbs. Related sysfs entries should be present:

    ls -d /sys/class/net/*/device/infiniband_verbs/uverbs* | cut -d / -f 5

    Example output:

  3. Optionally, retrieve their PCI bus addresses for whitelisting:

        for intf in eth2 eth3 eth4 eth5;
            (cd "/sys/class/net/${intf}/device/" && pwd -P);
    } |
    sed -n 's,.*/\(.*\),-w \1,p'

    Example output:

    -w 0000:83:00.0
    -w 0000:83:00.0
    -w 0000:84:00.0
    -w 0000:84:00.0


    There are only two distinct PCI bus addresses because the Mellanox ConnectX-3 adapters installed on this system are dual port.

  4. Request huge pages:

    echo 1024 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages/nr_hugepages
  5. Start testpmd with basic parameters:

    testpmd -c 0xff00 -n 4 -w 0000:83:00.0 -w 0000:84:00.0 -- --rxq=2 --txq=2 -i

    Example output:

    EAL: PCI device 0000:83:00.0 on NUMA socket 1
    EAL:   probe driver: 15b3:1007 librte_pmd_mlx4
    PMD: librte_pmd_mlx4: PCI information matches, using device "mlx4_0" (VF: false)
    PMD: librte_pmd_mlx4: 2 port(s) detected
    PMD: librte_pmd_mlx4: port 1 MAC address is 00:02:c9:b5:b7:50
    PMD: librte_pmd_mlx4: port 2 MAC address is 00:02:c9:b5:b7:51
    EAL: PCI device 0000:84:00.0 on NUMA socket 1
    EAL:   probe driver: 15b3:1007 librte_pmd_mlx4
    PMD: librte_pmd_mlx4: PCI information matches, using device "mlx4_1" (VF: false)
    PMD: librte_pmd_mlx4: 2 port(s) detected
    PMD: librte_pmd_mlx4: port 1 MAC address is 00:02:c9:b5:ba:b0
    PMD: librte_pmd_mlx4: port 2 MAC address is 00:02:c9:b5:ba:b1
    Interactive-mode selected
    Configuring Port 0 (socket 0)
    PMD: librte_pmd_mlx4: 0x867d60: TX queues number update: 0 -> 2
    PMD: librte_pmd_mlx4: 0x867d60: RX queues number update: 0 -> 2
    Port 0: 00:02:C9:B5:B7:50
    Configuring Port 1 (socket 0)
    PMD: librte_pmd_mlx4: 0x867da0: TX queues number update: 0 -> 2
    PMD: librte_pmd_mlx4: 0x867da0: RX queues number update: 0 -> 2
    Port 1: 00:02:C9:B5:B7:51
    Configuring Port 2 (socket 0)
    PMD: librte_pmd_mlx4: 0x867de0: TX queues number update: 0 -> 2
    PMD: librte_pmd_mlx4: 0x867de0: RX queues number update: 0 -> 2
    Port 2: 00:02:C9:B5:BA:B0
    Configuring Port 3 (socket 0)
    PMD: librte_pmd_mlx4: 0x867e20: TX queues number update: 0 -> 2
    PMD: librte_pmd_mlx4: 0x867e20: RX queues number update: 0 -> 2
    Port 3: 00:02:C9:B5:BA:B1
    Checking link statuses...
    Port 0 Link Up - speed 10000 Mbps - full-duplex
    Port 1 Link Up - speed 40000 Mbps - full-duplex
    Port 2 Link Up - speed 10000 Mbps - full-duplex
    Port 3 Link Up - speed 40000 Mbps - full-duplex