OpenStack for NFV Applications: SR-IOV and PCI Passthrough



Network Function Virtualisation (NFV) initiatives in the telecommunication industry require specific OpenStack functionalities enabled.

Without entering into the details of the NFV specifications, the goal in OpenStack is to optimise network, memory and CPU performance on the running instances.

In this article we’ll see Single Root I/O Virtualisation (SR-IOV) and PCI-Passthrough, which are commonly required by some Virtual Network Functions (VNF) running as instances on top of OpenStack.

In addition to SR-IOV and PCI-Passthrough there are other techniques such as DPDK, CPU pinning and the use of NUMA nodes which also are usually required by VNFs. A future post will cover some of them.


SR-IOV allows a PCIe network interface, offering Physical Functions (PF) to expose multiple network interfaces, appearing as Virtual Functions (VF). For example, the network interface p5p1 configured with 5 VFs looks like this from the operating system:

# ip link show p5p1
8: p5p1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT qlen 1000
 link/ether a0:36:9f:8f:3f:b8 brd ff:ff:ff:ff:ff:ff
 vf 0 MAC 00:00:00:00:00:00, spoof checking on, link-state auto
 vf 1 MAC 00:00:00:00:00:00, spoof checking on, link-state auto
 vf 2 MAC 00:00:00:00:00:00, spoof checking on, link-state auto
 vf 3 MAC 00:00:00:00:00:00, spoof checking on, link-state auto
 vf 4 MAC 00:00:00:00:00:00, spoof checking on, link-state auto

The VFs can be used by the OS or exposed to VMs. They look exactly as regular NIC:

# ip link show p5p1_1
18: p5p1_1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 1000
 link/ether 72:1c:ef:b0:a8:d0 brd ff:ff:ff:ff:ff:ff

Only certain NICs support SR-IOV. In this example I’m using Intel’s X540-AT2 NICs which uses the driver ixgbe.

Linux configuration for SR-IOV

To use SR-IOV in OpenStack, firstly we need to make sure the operating system is configured to support it. There are 2 kernel parameters to be set:


Note that ixgbe is specific for the Intel X540-AT2 NIC and you might be using another one. You can also use a different number of VFs.

To enable the parameters in RHEL based systems it works as follows:

  1. Add the parameters to /etc/default/grub in GRUB_CMDLINE_LINUX
  2. Regenerate the config file with: grub2-mkconfig -o /boot/grub2/grub.cfg
  3. Rebuild the initramfs file with: dracut -f -v

We also need to make sure that the admin state of the interface is UP:

# ip link show p5p1
# ip link set p5p1 up

And by setting the appropriate network interface configuration file in /etc/sysconfig/network-scripts/ifcfg-p5p1 as:


OpenStack configuration for SR-IOV

1. Neutron

SR-IOV works with the VLAN type driver in Neutron. We enable it in /etc/neutron/plugin.ini:


The mechanism driver is sriovnicswitch, which is configured in the same [ml2] section as follows:


Every time we create a new SR-IOV network in Neutron, it will configure it on a VLAN from a range that we need specify. It needs a name too. In this example the range is 1010 to 1020 and the physical network for Neutron will be called physnet_sriov :


Now, we configure SR-IOV settings in /etc/neutron/plugins/ml2/ml2_conf_sriov.ini. In the section [ml2_sriov] we need to tell the driver which NIC we will use:


The numbers represent the vendor ID (8086) and the product ID (1515).  To get them we can use lspci -nn:

# lspci -nn|grep X540-AT2
06:00.0 Ethernet controller [0200]: Intel Corporation Ethernet Controller 10-Gigabit X540-AT2 [8086:1528] (rev 01)
06:00.1 Ethernet controller [0200]: Intel Corporation Ethernet Controller 10-Gigabit X540-AT2 [8086:1528] (rev 01)

By default the neutron-server service is not loading the configuration in the file ml2_conf_sriov.ini so we need to add it to its systemd service in /usr/lib/systemd/system/neutron-server.service:

ExecStart=/usr/bin/neutron-server --config-file /usr/share/neutron/neutron-dist.conf --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugin.ini --config-file /etc/neutron/plugins/ml2/ml2_conf_sriov.ini  --log-file /var/log/neutron/server.log 

And after that restart the service:

# systemctl restart neutron-server

2. Nova scheduler

We need to tell the Nova scheduler about the SR-IOV so that it can schedule instances to compute nodes with SR-IOV support.

In the [DEFAULT] section of /etc/nova/nova.conf adding the PciPassthroughFilter. Also ensure scheduler_available_filters is set as follows:


And restart Nova scheduler:

# systemctl restart openstack-nova-scheduler

3. Nova compute

Nova compute needs to know which PFs can be used for SR-IOV so that VFs are exposed – actually via PCI-passthrough – to the instances. Also, it needs to know that when we create a network with Neutron specifying the physical network physnet_sriov  – configured before in Neutron with network_vlan_ranges – it will use the SR-IOV NIC.

That’s done by the config flag pci_passthrough_whitelist in /etc/nova/nova.conf:

pci_passthrough_whitelist = {"devname": "p5p1", "physical_network": "physnet_sriov"}

And simply restart Nova compute:

# systemctl restart openstack-nova-compute

4. SR-IOV NIC agent

We can optionally configure the SR-IOV NIC agent to manage the admin state of the NICs. When a VF NIC is used by an instance and then released, sometimes the NIC goes into DOWN state and the admin manually has to bring it back to UP state. There’s an article that describes how to do this in the official Red Hat documentation:

Enable the OpenStack Networking SR-IOV agent

Not all the drivers work with the agent and that was the case for the Intel X540-AT2 NIC.

Creating OpenStack instances with a SR-IOV port

1. Create the network

We configured the physnet_sriov network in Neutron to use the SR-IOV interface p5p1. Let’s create the network and its subnet in Neutron now:

$ neutron net-create nfv_sriov --shared --provider:network_type vlan --provider:physical_network physnet_sriov
$ neutron subnet-create --name nfv_subnet_sriov --disable-dhcp --allocation-pool start=,end= nfv_sriov

Remember we configured a VLAN range, so Neutron will choose a VLAN from it, but if we wanted to specify one we can by using –provider:segmentation_id=1010 when creating the network.

2. Create the port

We’ll pass a port to the instance instead of the nfv_sriov network. To create it we do this:

$ neutron port-create nfv_sriov --name sriov-port --binding:vnic-type direct

Save the ID of the port as we’ll need it for creating the instance.

3. Create the instance

We will now create an instance that uses two NICs, one created the standard way – in a private network which already existed in Neutron – and a another one with the port created before. Assuming  SRIOV_PORT_ID is the ID of the port and PRIVATE_NETWORK_ID is the ID of the pre-existing private network, this is how we create it:

$ openstack server create --flavor m1.small --nic port-id=$SRIOV_PORT_ID --nic net-id=$PRIVATE_NETWORK_ID --image centos7 sr-iov-instance1

If you have key-pairs or other options  you use, pass them too in the openstack server create command.

Log in the instance as usual and you’ll notice two interfaces, eth0 and probably ens5, which is the SR-IOV NIC ready to be used.

Note as well that one of the VFs has now the same MAC address than the Neutron port we created above:

$ ip link show p5p1
8: p5p1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT qlen 1000
    link/ether a0:36:9f:8b:cd:80 brd ff:ff:ff:ff:ff:ff
    vf 0 MAC 00:00:00:00:00:00, spoof checking on, link-state auto
    vf 1 MAC 00:00:00:00:00:00, spoof checking on, link-state auto
    vf 2 MAC 00:00:00:00:00:00, spoof checking on, link-state auto
    vf 3 MAC 00:00:00:00:00:00, spoof checking on, link-state auto
    vf 4 MAC fa:16:3e:e0:3f:be, spoof checking on, link-state auto


If our VNF (or any virtualised application for that matter) required direct access to a PCI interface in the hypervisor, the PCI-Passthrough functionality in Libvirt/KVM and OpenStack allows us doing it. This is also common in High Performance Computing (HPC), not only with NIC interfaces but, for example, sharing GPUs with the instances.

In this example we’ll use another NIC interface to pass it to the instance: p5p2 in the hypervisor.

Linux configuration for PCI-Passthrough

First, just like before, make sure the admin state if the interface is UP so let’s do the same:

# ip link show p5p2
# ip link set p5p2 up

And in /etc/sysconfig/network-scripts/ifcfg-p5p2:


The kernel options are the same ones we used above so nothing else is required at this point.

OpenStack configuration for PCI-Passthrough

Nova scheduler is already configured for PCI-Passthrough so only Nova compute needs to be made aware of the device we want to pass through.

1. Nova compute

We need a second entry in /etc/nova/nova.conf with pci_passthrough_whitelist. This will tell Nova compute that the interface p5p2 can be taken from the Linux OS and passed into an instance:

pci_passthrough_whitelist={ "devname": "p5p2" }

Now, we need to tag this interface with a name that will be used by Nova during the creation of the instance. For example we can call it my_PF. This is also done in the /etc/nova/nova.conf file:

pci_alias={ "vendor_id": "8086", "product_id": "1528", "name": "my_PF"}

Note that the vendor and product IDs are the same ones than before as both NICs are the same. Again, you can get your PCI device IDs with lspci -nn.

2. Nova flavor

The way OpenStack has been designed to allow passing PCI devices to instances is via flavors. The tag we used before (my_PF) needs to be associated with a new flavor in this way:

$ openstack flavor create --ram 4096 --disk 100 --vcpus 2 m1.medium.pci_passthrough
$ openstack flavor set --property "pci_passthrough:alias"="my_PF:1" m1.medium.pci_passthrough

3. Create the instance

Now all we need to do is launching an instance using this new flavor and it will automatically be configured by Nova compute – and then by Libvirt – with the PCI device in it.

$ openstack server create --flavor m1.medium.pci_passthrough --nic net-id=$PRIVATE_NETWORK_ID --image centos7 pci-passthrough-instance1

Again, if you have more options you need such as key-pairs or adding later a floating IP to access the instance you can do it too.

After that, the instance will show again an interface ens5 which is the p5p2 interface. In addition the interface p5p2 will disappear from the operating system while the instance exists.