CDI - The Container Device Interface

NOTE: The API for injecting CDI devices that existed at container-device-interface/pkg has been removed. Users of this API should migrate to the one at container-device-interface/pkg/cdi as this is actively maintained.

What is CDI?

CDI (Container Device Interface), is a specification, for container-runtimes, to support third-party devices.

It introduces an abstract notion of a device as a resource. Such devices are uniquely specified by a fully-qualified name that is constructed from a vendor ID, a device class, and a name that is unique per vendor ID-device class pair.

vendor.com/class=unique_name

The combination of vendor ID and device class (vendor.com/class in the above example) is referred to as the device kind.

CDI concerns itself only with enabling containers to be device aware. Areas like resource management are explicitly left out of CDI (and are expected to be handled by the orchestrator). Because of this focus, the CDI specification is simple to implement and allows great flexibility for runtimes and orchestrators.

Note: The CDI model is based on the Container Networking Interface (CNI) model and specification.

Why is CDI needed?

On Linux, enabling a container to be device aware used to be as simple as exposing a device node in that container. However, as devices and software grows more complex, vendors want to perform more operations, such as:

• Exposing a device to a container can require exposing more than one device node, mounting files from the runtime namespace, or hiding procfs entries.
• Performing compatibility checks between the container and the device (e.g: Can this container run on this device?).
• Performing runtime-specific operations (e.g: VM vs Linux container-based runtimes).
• Performing device-specific operations (e.g: scrubbing the memory of a GPU or reconfiguring an FPGA).

In the absence of a standard for third-party devices, vendors often have to write and maintain multiple plugins for different runtimes or even directly contribute vendor-specific code in the runtime. Additionally, runtimes don't uniformly expose a plugin system (or even expose a plugin system at all) leading to duplication of the functionality in higher-level abstractions (such as Kubernetes device plugins).

How does CDI work?

For CDI to work the following needs to be done:

• CDI file containing updates for the OCI spec in JSON or YAML format (with a .json or .yaml file extension, respectively) should be present in a CDI spec directory. The default directories are /etc/cdi and /var/run/cdi, but may depend on your runtime configuration
• One or more fully-qualified device names should be passed to the runtime either using command line parameters for CLI tools such as podman or Docker, or using container annotations or CRI fields for CRI-O and containerd
• The container runtime should be able to find the CDI file by the device name and update the container config using CDI file content.

How to configure CDI?

CRI-O configuration

In CRI-O CDI support is enabled by default. It is configured with the default /etc/cdi, /var/run/cdi CDI directory locations. Therefore, you can start using CDI simply by dropping CDI configuration files in either of those directories, static configuration into /etc/cdi and dynamically updated one into /var/run/cdi. If you are unsure of the configured directories you can run this command to find them out:

$ crio config |& grep -B1 -A5 cdi_spec_dirs

containerd configuration

To enable and configure CDI support in the containerd runtime 2 configuration options enable_cdi and cdi_spec_dirs should be set in the plugins."io.containerd.grpc.v1.cri section of the containerd configuration file (/etc/containerd/config.toml by default):

[plugins."io.containerd.grpc.v1.cri"]
  enable_cdi = true
  cdi_spec_dirs = ["/etc/cdi", "/var/run/cdi"]

Remember to restart containerd for any configuration changes to take effect.

Docker configuration

To enable and configure CDI support in the Docker Daemon Docker 25 or later is required.

In addition, the CDI feature must be enabled. That means including the following in the daemon config (/etc/docker/daemon.json by default):

{
  "features": {
    "cdi": true
  }
}

Remember to restart to Docker daemon for any configuration changes to take effect.

Podman configuration

podman does not require any specific configuration to enable CDI support and processes specified --device flags directly. If fully-qualified device selectors (e.g. vendor.com/device=myDevice) are included the CDI specifications at the default location (/etc/cdi and /var/run/cdi) are checked for matching devices.

Note: Although initial support was added in v3.2.0 this was updated for the tagged v0.3.0 CDI spec in v4.1.0-rc.1 with commit a234e4e.

Examples

Full-blown CDI specification

$ mkdir /etc/cdi
$ cat > /etc/cdi/vendor.json <<EOF
{
  "cdiVersion": "0.6.0",
  "kind": "vendor.com/device",
  "devices": [
    {
      "name": "myDevice",
      "containerEdits": {
        "deviceNodes": [
          {"hostPath": "/vendor/dev/card1", "path": "/dev/card1", "type": "c", "major": 25, "minor": 25, "fileMode": 384, "permissions": "rw", "uid": 1000, "gid": 1000},
          {"path": "/dev/card-render1", "type": "c", "major": 25, "minor": 25, "fileMode": 384, "permissions": "rwm", "uid": 1000, "gid": 1000}
        ]
      }
    }
  ],
  "containerEdits": {
    "env": [
      "FOO=VALID_SPEC",
      "BAR=BARVALUE1"
    ],
    "deviceNodes": [
      {"path": "/dev/vendorctl", "type": "b", "major": 25, "minor": 25, "fileMode": 384, "permissions": "rw", "uid": 1000, "gid": 1000}
    ],
    "mounts": [
      {"hostPath": "/bin/vendorBin", "containerPath": "/bin/vendorBin"},
      {"hostPath": "/usr/lib/libVendor.so.0", "containerPath": "/usr/lib/libVendor.so.0"},
      {"hostPath": "tmpfs", "containerPath": "/tmp/data", "type": "tmpfs", "options": ["nosuid","strictatime","mode=755","size=65536k"]}
    ],
    "hooks": [
      {"createContainer": {"path": "/bin/vendor-hook"} },
      {"startContainer": {"path": "/usr/bin/ldconfig"} }
    ]
  }
}
EOF

Assuming this specification has been generated and is available in either /etc/cdi or /var/run/cdi (or wherever a CDI-enabled consumer is configured to read CDI specifications from), the devices can be accessed through their fully-qualified device names.

For example, in the case of podman the CLI for accessing the device would be:

$ podman run --device vendor.com/device=myDevice ...

Using Annotations per device to add meta-information

$ mkdir /etc/cdi
$ cat > /etc/cdi/vendor-annotations.json <<EOF
{
  "cdiVersion": "0.6.0",
  "kind": "vendor.com/device",
  "devices": [
    {
      "name": "myDevice",
      "annotations": {
        "whatever": "false"
        "whenever": "true"
      }
      "containerEdits": {
        "deviceNodes": [
          {"path": "/dev/vfio/71"}
        ]
      }
    }
  ]
}
EOF

Issues and Contributing

Check out the Contributing document!

• Please let us know by filing a new issue
• You can contribute by opening a pull request