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Kata Containers Architecture

Overview

This is an architectural overview of Kata Containers, based on the 1.5.0 release.

The two primary deliverables of the Kata Containers project are a container runtime and a CRI friendly shim. There is also a CRI friendly library API behind them.

The Kata Containers runtime (kata-runtime) is compatible with the OCI runtime specification and therefore works seamlessly with the Docker* Engine pluggable runtime architecture. It also supports the Kubernetes* Container Runtime Interface (CRI) through the CRI-O* and Containerd CRI Plugin* implementation. In other words, you can transparently select between the default Docker and CRI shim runtime (runc) and kata-runtime.

kata-runtime creates a QEMU*/KVM virtual machine for each container or pod, the Docker engine or kubelet (Kubernetes) creates respectively.

Docker and Kata Containers

The containerd-shim-kata-v2 (shown as shimv2 from this point onwards) is another Kata Containers entrypoint, which implements the Containerd Runtime V2 (Shim API) for Kata. With shimv2, Kubernetes can launch Pod and OCI compatible containers with one shim (the shimv2) per Pod instead of 2N+1 shims (a containerd-shim and a kata-shim for each container and the Pod sandbox itself), and no standalone kata-proxy process even if no VSOCK is available.

Kubernetes integration with shimv2

The container process is then spawned by agent, an agent process running as a daemon inside the virtual machine. kata-agent runs a gRPC server in the guest using a VIRTIO serial or VSOCK interface which QEMU exposes as a socket file on the host. kata-runtime uses a gRPC protocol to communicate with the agent. This protocol allows the runtime to send container management commands to the agent. The protocol is also used to carry the I/O streams (stdout, stderr, stdin) between the containers and the manage engines (e.g. Docker Engine).

For any given container, both the init process and all potentially executed commands within that container, together with their related I/O streams, need to go through the VIRTIO serial or VSOCK interface exported by QEMU. In the VIRTIO serial case, a Kata Containers proxy (kata-proxy) instance is launched for each virtual machine to handle multiplexing and demultiplexing those commands and streams.

On the host, each container process's removal is handled by a reaper in the higher layers of the container stack. In the case of Docker or containerd it is handled by containerd-shim. In the case of CRI-O it is handled by conmon. For clarity, for the remainder of this document the term "container process reaper" will be used to refer to either reaper. As Kata Containers processes run inside their own virtual machines, the container process reaper cannot monitor, control or reap them. kata-runtime fixes that issue by creating an additional shim process (kata-shim) between the container process reaper and kata-proxy. A kata-shim instance will both forward signals and stdin streams to the container process on the guest and pass the container stdout and stderr streams back up the stack to the CRI shim or Docker via the container process reaper. kata-runtime creates a kata-shim daemon for each container and for each OCI command received to run within an already running container (example, docker exec).

Since Kata Containers version 1.5, the new introduced shimv2 has integrated the functionalities of the reaper, the kata-runtime, the kata-shim, and the kata-proxy. As a result, there will not be any of the additional processes previously listed.

The container workload, that is, the actual OCI bundle rootfs, is exported from the host to the virtual machine. In the case where a block-based graph driver is configured, virtio-scsi will be used. In all other cases a 9pfs VIRTIO mount point will be used. kata-agent uses this mount point as the root filesystem for the container processes.

Virtualization

How Kata Containers maps container concepts to virtual machine technologies, and how this is realized in the multiple hypervisors and VMMs that Kata supports is described within the virtualization documentation

Guest assets

The hypervisor will launch a virtual machine which includes a minimal guest kernel and a guest image.

Guest kernel

The guest kernel is passed to the hypervisor and used to boot the virtual machine. The default kernel provided in Kata Containers is highly optimized for kernel boot time and minimal memory footprint, providing only those services required by a container workload. This is based on a very current upstream Linux kernel.

Guest image

Kata Containers supports both an initrd and rootfs based minimal guest image.

Root filesystem image

The default packaged root filesystem image, sometimes referred to as the "mini O/S", is a highly optimized container bootstrap system based on Clear Linux. It provides an extremely minimal environment and has a highly optimized boot path.

The only services running in the context of the mini O/S are the init daemon (systemd) and the Agent. The real workload the user wishes to run is created using libcontainer, creating a container in the same manner that is done by runc.

For example, when docker run -ti ubuntu date is run:

  • The hypervisor will boot the mini-OS image using the guest kernel.
  • systemd, running inside the mini-OS context, will launch the kata-agent in the same context.
  • The agent will create a new confined context to run the specified command in (date in this example).
  • The agent will then execute the command (date in this example) inside this new context, first setting the root filesystem to the expected Ubuntu* root filesystem.

Initrd image

A compressed cpio(1) archive, created from a rootfs which is loaded into memory and used as part of the Linux startup process. During startup, the kernel unpacks it into a special instance of a tmpfs that becomes the initial root filesystem.

The only service running in the context of the initrd is the Agent as the init daemon. The real workload the user wishes to run is created using libcontainer, creating a container in the same manner that is done by runc.

Agent

kata-agent is a process running in the guest as a supervisor for managing containers and processes running within those containers.

The kata-agent execution unit is the sandbox. A kata-agent sandbox is a container sandbox defined by a set of namespaces (NS, UTS, IPC and PID). kata-runtime can run several containers per VM to support container engines that require multiple containers running inside a pod. In the case of docker, kata-runtime creates a single container per pod.

kata-agent communicates with the other Kata components over gRPC. It also runs a yamux server on the same gRPC URL.

The kata-agent makes use of libcontainer to manage the lifecycle of the container. This way the kata-agent reuses most of the code used by runc.

Agent gRPC protocol

placeholder

Runtime

kata-runtime is an OCI compatible container runtime and is responsible for handling all commands specified by the OCI runtime specification and launching kata-shim instances.

kata-runtime heavily utilizes the virtcontainers project, which provides a generic, runtime-specification agnostic, hardware-virtualized containers library.

Configuration

The runtime uses a TOML format configuration file called configuration.toml. By default this file is installed in the /usr/share/defaults/kata-containers directory and contains various settings such as the paths to the hypervisor, the guest kernel and the mini-OS image.

Most users will not need to modify the configuration file.

The file is well commented and provides a few "knobs" that can be used to modify the behavior of the runtime.

The configuration file is also used to enable runtime debug output.

Significant OCI commands

Here we describe how kata-runtime handles the most important OCI commands.

create

When handling the OCI create command, kata-runtime goes through the following steps:

  1. Create the network namespace where we will spawn VM and shims processes.
  2. Call into the pre-start hooks. One of them should be responsible for creating the veth network pair between the host network namespace and the network namespace freshly created.
  3. Scan the network from the new network namespace, and create a MACVTAP connection between the veth interface and a tap interface into the VM.
  4. Start the VM inside the network namespace by providing the tap interface previously created.
  5. Wait for the VM to be ready.
  6. Start kata-proxy, which will connect to the created VM. The kata-proxy process will take care of proxying all communications with the VM. Kata has a single proxy per VM.
  7. Communicate with kata-agent (through the proxy) to configure the sandbox inside the VM.
  8. Communicate with kata-agent to create the container, relying on the OCI configuration file config.json initially provided to kata-runtime. This spawns the container process inside the VM, leveraging the libcontainer package.
  9. Start kata-shim, which will connect to the gRPC server socket provided by the kata-proxy. kata-shim will spawn a few Go routines to parallelize blocking calls ReadStdout() , ReadStderr() and WaitProcess(). Both ReadStdout() and ReadStderr() are run through infinite loops since kata-shim wants the output of those until the container process terminates. WaitProcess() is a unique call which returns the exit code of the container process when it terminates inside the VM. Note that kata-shim is started inside the network namespace, to allow upper layers to determine which network namespace has been created and by checking the kata-shim process. It also creates a new PID namespace by entering into it. This ensures that all kata-shim processes belonging to the same container will get killed when the kata-shim representing the container process terminates.

At this point the container process is running inside of the VM, and it is represented on the host system by the kata-shim process.

kata-oci-create

start

With traditional containers, start launches a container process in its own set of namespaces. With Kata Containers, the main task of kata-runtime is to ask kata-agent to start the container workload inside the virtual machine. kata-runtime will run through the following steps:

  1. Communicate with kata-agent (through the proxy) to start the container workload inside the VM. If, for example, the command to execute inside of the container is top, the kata-shim's ReadStdOut() will start returning text output for top, and WaitProcess() will continue to block as long as the top process runs.
  2. Call into the post-start hooks. Usually, this is a no-op since nothing is provided (this needs clarification)

kata-oci-start

exec

OCI exec allows you to run an additional command within an already running container. In Kata Containers, this is handled as follows:

  1. A request is sent to the kata agent (through the proxy) to start a new process inside an existing container running within the VM.
  2. A new kata-shim is created within the same network and PID namespaces as the original kata-shim representing the container process. This new kata-shim is used for the new exec process.

Now the process started with exec is running within the VM, sharing uts, pid, mnt and ipc namespaces with the container process.

kata-oci-exec

kill

When sending the OCI kill command, the container runtime should send a UNIX signal to the container process. A kill sending a termination signal such as SIGKILL or SIGTERM is expected to terminate the container process. In the context of a traditional container, this means stopping the container. For kata-runtime, this translates to stopping the container and the VM associated with it.

  1. Send a request to kill the container process to the kata-agent (through the proxy).
  2. Wait for kata-shim process to exit.
  3. Force kill the container process if kata-shim process didn't return after a timeout. This is done by communicating with kata-agent (connecting the proxy), sending SIGKILL signal to the container process inside the VM.
  4. Wait for kata-shim process to exit, and return an error if we reach the timeout again.
  5. Communicate with kata-agent (through the proxy) to remove the container configuration from the VM.
  6. Communicate with kata-agent (through the proxy) to destroy the sandbox configuration from the VM.
  7. Stop the VM.
  8. Remove all network configurations inside the network namespace and delete the namespace.
  9. Execute post-stop hooks.

If kill was invoked with a non-termination signal, this simply signals the container process. Otherwise, everything has been torn down, and the VM has been removed.

delete

delete removes all internal resources related to a container. A running container cannot be deleted unless the OCI runtime is explicitly being asked to, by using --force flag.

If the sandbox is not stopped, but the particular container process returned on its own already, the kata-runtime will first go through most of the steps a kill would go through for a termination signal. After this process, or if the sandboxID was already stopped to begin with, then kata-runtime will:

  1. Remove container resources. Every file kept under /var/{lib,run}/virtcontainers/sandboxes/<sandboxID>/<containerID>.
  2. Remove sandbox resources. Every file kept under /var/{lib,run}/virtcontainers/sandboxes/<sandboxID>.

At this point, everything related to the container should have been removed from the host system, and no related process should be running.

state

state returns the status of the container. For kata-runtime, this means being able to detect if the container is still running by looking at the state of kata-shim process representing this container process.

  1. Ask the container status by checking information stored on disk. (clarification needed)
  2. Check kata-shim process representing the container.
  3. In case the container status on disk was supposed to be ready or running, and the kata-shim process no longer exists, this involves the detection of a stopped container. This means that before returning the container status, the container has to be properly stopped. Here are the steps involved in this detection:
    1. Wait for kata-shim process to exit.
    2. Force kill the container process if kata-shim process didn't return after a timeout. This is done by communicating with kata-agent (connecting the proxy), sending SIGKILL signal to the container process inside the VM.
    3. Wait for kata-shim process to exit, and return an error if we reach the timeout again.
    4. Communicate with kata-agent (connecting the proxy) to remove the container configuration from the VM.
  4. Return container status.

Proxy

Communication with the VM can be achieved by either virtio-serial or, if the host kernel is newer than v4.8, a virtual socket, vsock can be used. The default is virtio-serial.

The VM will likely be running multiple container processes. In the event virtio-serial is used, the I/O streams associated with each process needs to be multiplexed and demultiplexed on the host. On systems with vsock support, this component becomes optional.

kata-proxy is a process offering access to the VM kata-agent to multiple kata-shim and kata-runtime clients associated with the VM. Its main role is to route the I/O streams and signals between each kata-shim instance and the kata-agent. kata-proxy connects to kata-agent on a Unix domain socket that kata-runtime provides while spawning kata-proxy. kata-proxy uses yamux to multiplex gRPC requests on its connection to the kata-agent.

When proxy type is configured as proxyBuiltIn, we do not spawn a separate process to proxy gRPC connections. Instead a built-in Yamux gRPC dialer is used to connect directly to kata-agent. This is used by CRI container runtime server frakti which calls directly into kata-runtime.

Shim

A container process reaper, such as Docker's containerd-shim or CRI-O's conmon, is designed around the assumption that it can monitor and reap the actual container process. As the container process reaper runs on the host, it cannot directly monitor a process running within a virtual machine. At most it can see the QEMU process, but that is not enough. With Kata Containers, kata-shim acts as the container process that the container process reaper can monitor. Therefore kata-shim needs to handle all container I/O streams (stdout, stdin and stderr) and forward all signals the container process reaper decides to send to the container process.

kata-shim has an implicit knowledge about which VM agent will handle those streams and signals and thus acts as an encapsulation layer between the container process reaper and the kata-agent. kata-shim:

  • Connects to kata-proxy on a Unix domain socket. The socket URL is passed from kata-runtime to kata-shim when the former spawns the latter along with a containerID and execID. The containerID and execID are used to identify the true container process that the shim process will be shadowing or representing.
  • Forwards the standard input stream from the container process reaper into kata-proxy using gRPC WriteStdin gRPC API.
  • Reads the standard output/error from the container process.
  • Forwards signals it receives from the container process reaper to kata-proxy using SignalProcessRequest API.
  • Monitors terminal changes and forwards them to kata-proxy using gRPC TtyWinResize API.

Networking

Containers will typically live in their own, possibly shared, networking namespace. At some point in a container lifecycle, container engines will set up that namespace to add the container to a network which is isolated from the host network, but which is shared between containers

In order to do so, container engines will usually add one end of a virtual ethernet (veth) pair into the container networking namespace. The other end of the veth pair is added to the host networking namespace.

This is a very namespace-centric approach as many hypervisors (in particular QEMU) cannot handle veth interfaces. Typically, TAP interfaces are created for VM connectivity.

To overcome incompatibility between typical container engines expectations and virtual machines, kata-runtime networking transparently connects veth interfaces with TAP ones using MACVTAP:

Kata Containers networking

Kata Containers supports both CNM and CNI for networking management.

CNM

High-level CNM Diagram

CNM lifecycle

  1. RequestPool

  2. CreateNetwork

  3. RequestAddress

  4. CreateEndPoint

  5. CreateContainer

  6. Create config.json

  7. Create PID and network namespace

  8. ProcessExternalKey

  9. JoinEndPoint

  10. LaunchContainer

  11. Launch

  12. Run container

Detailed CNM Diagram

Runtime network setup with CNM

  1. Read config.json

  2. Create the network namespace

  3. Call the prestart hook (from inside the netns)

  4. Scan network interfaces inside netns and get the name of the interface created by prestart hook

  5. Create bridge, TAP, and link all together with network interface previously created

Network Hotplug

Kata Containers has developed a set of network sub-commands and APIs to add, list and remove a guest network endpoint and to manipulate the guest route table.

The following diagram illustrates the Kata Containers network hotplug workflow.

Network Hotplug

Storage

Container workloads are shared with the virtualized environment through 9pfs. The devicemapper storage driver is a special case. The driver uses dedicated block devices rather than formatted filesystems, and operates at the block level rather than the file level. This knowledge is used to directly use the underlying block device instead of the overlay file system for the container root file system. The block device maps to the top read-write layer for the overlay. This approach gives much better I/O performance compared to using 9pfs to share the container file system.

The approach above does introduce a limitation in terms of dynamic file copy in/out of the container using the docker cp operations. The copy operation from host to container accesses the mounted file system on the host-side. This is not expected to work and may lead to inconsistencies as the block device will be simultaneously written to from two different mounts. The copy operation from container to host will work, provided the user calls sync(1) from within the container prior to the copy to make sure any outstanding cached data is written to the block device.

docker cp [OPTIONS] CONTAINER:SRC_PATH HOST:DEST_PATH
docker cp [OPTIONS] HOST:SRC_PATH CONTAINER:DEST_PATH

Kata Containers has the ability to hotplug and remove block devices, which makes it possible to use block devices for containers started after the VM has been launched.

Users can check to see if the container uses the devicemapper block device as its rootfs by calling mount(8) within the container. If the devicemapper block device is used, / will be mounted on /dev/vda. Users can disable direct mounting of the underlying block device through the runtime configuration.

Kubernetes support

Kubernetes* is a popular open source container orchestration engine. In Kubernetes, a set of containers sharing resources such as networking, storage, mount, PID, etc. is called a Pod. A node can have multiple pods, but at a minimum, a node within a Kubernetes cluster only needs to run a container runtime and a container agent (called a Kubelet).

A Kubernetes cluster runs a control plane where a scheduler (typically running on a dedicated master node) calls into a compute Kubelet. This Kubelet instance is responsible for managing the lifecycle of pods within the nodes and eventually relies on a container runtime to handle execution. The Kubelet architecture decouples lifecycle management from container execution through the dedicated gRPC based Container Runtime Interface (CRI).

In other words, a Kubelet is a CRI client and expects a CRI implementation to handle the server side of the interface. CRI-O* and Containerd CRI Plugin* are CRI implementations that rely on OCI compatible runtimes for managing container instances.

Kata Containers is an officially supported CRI-O and Containerd CRI Plugin runtime. It is OCI compatible and therefore aligns with project's architecture and requirements. However, due to the fact that Kubernetes execution units are sets of containers (also known as pods) rather than single containers, the Kata Containers runtime needs to get extra information to seamlessly integrate with Kubernetes.

Problem statement

The Kubernetes* execution unit is a pod that has specifications detailing constraints such as namespaces, groups, hardware resources, security contents, etc shared by all the containers within that pod. By default the Kubelet will send a container creation request to its CRI runtime for each pod and container creation. Without additional metadata from the CRI runtime, the Kata Containers runtime will thus create one virtual machine for each pod and for each containers within a pod. However the task of providing the Kubernetes pod semantics when creating one virtual machine for each container within the same pod is complex given the resources of these virtual machines (such as networking or PID) need to be shared.

The challenge with Kata Containers when working as a Kubernetes* runtime is thus to know when to create a full virtual machine (for pods) and when to create a new container inside a previously created virtual machine. In both cases it will get called with very similar arguments, so it needs the help of the Kubernetes CRI runtime to be able to distinguish a pod creation request from a container one.

Containerd

As of Kata Containers 1.5, using shimv2 with containerd 1.2.0 or above is the preferred way to run Kata Containers with Kubernetes (see the howto). The CRI-O will catch up soon (kubernetes-sigs/cri-o#2024).

Refer to the following how-to guides:

CRI-O

OCI annotations

In order for the Kata Containers runtime (or any virtual machine based OCI compatible runtime) to be able to understand if it needs to create a full virtual machine or if it has to create a new container inside an existing pod's virtual machine, CRI-O adds specific annotations to the OCI configuration file (config.json) which is passed to the OCI compatible runtime.

Before calling its runtime, CRI-O will always add a io.kubernetes.cri-o.ContainerType annotation to the config.json configuration file it produces from the Kubelet CRI request. The io.kubernetes.cri-o.ContainerType annotation can either be set to sandbox or container. Kata Containers will then use this annotation to decide if it needs to respectively create a virtual machine or a container inside a virtual machine associated with a Kubernetes pod:

	containerType, err := ociSpec.ContainerType()
	if err != nil {
		return err
	}

	handleFactory(ctx, runtimeConfig)

	disableOutput := noNeedForOutput(detach, ociSpec.Process.Terminal)

	var process vc.Process
	switch containerType {
	case vc.PodSandbox:
		process, err = createSandbox(ctx, ociSpec, runtimeConfig, containerID, bundlePath, console, disableOutput, systemdCgroup)
		if err != nil {
			return err
		}
	case vc.PodContainer:
		process, err = createContainer(ctx, ociSpec, containerID, bundlePath, console, disableOutput)
		if err != nil {
			return err
		}
	}

Mixing VM based and namespace based runtimes

Note: Since Kubernetes 1.12, the Kubernetes RuntimeClass has been supported and the user can specify runtime without the non-standardized annotations.

One interesting evolution of the CRI-O support for kata-runtime is the ability to run virtual machine based pods alongside namespace ones. With CRI-O and Kata Containers, one can introduce the concept of workload trust inside a Kubernetes cluster.

A cluster operator can now tag (through Kubernetes annotations) container workloads as trusted or untrusted. The former labels known to be safe workloads while the latter describes potentially malicious or misbehaving workloads that need the highest degree of isolation. In a software development context, an example of a trusted workload would be a containerized continuous integration engine whereas all developers applications would be untrusted by default. Developers workloads can be buggy, unstable or even include malicious code and thus from a security perspective it makes sense to tag them as untrusted. A CRI-O and Kata Containers based Kubernetes cluster handles this use case transparently as long as the deployed containers are properly tagged. All untrusted containers will be handled by Kata Containers and thus run in a hardware virtualized secure sandbox while runc, for example, could handle the trusted ones.

CRI-O's default behavior is to trust all pods, except when they're annotated with io.kubernetes.cri-o.TrustedSandbox set to false. The default CRI-O trust level is set through its configuration.toml configuration file. Generally speaking, the CRI-O runtime selection between its trusted runtime (typically runc) and its untrusted one (kata-runtime) is a function of the pod Privileged setting, the io.kubernetes.cri-o.TrustedSandbox annotation value, and the default CRI-O trust level. When a pod is Privileged, the runtime will always be runc. However, when a pod is not Privileged the runtime selection is done as follows:

io.kubernetes.cri-o.TrustedSandbox not set io.kubernetes.cri-o.TrustedSandbox = true io.kubernetes.cri-o.TrustedSandbox = false
Default CRI-O trust level: trusted runc runc Kata Containers
Default CRI-O trust level: untrusted Kata Containers Kata Containers Kata Containers

Appendices

DAX

Kata Containers utilizes the Linux kernel DAX (Direct Access filesystem) feature to efficiently map some host-side files into the guest VM space. In particular, Kata Containers uses the QEMU NVDIMM feature to provide a memory-mapped virtual device that can be used to DAX map the virtual machine's root filesystem into the guest memory address space.

Mapping files using DAX provides a number of benefits over more traditional VM file and device mapping mechanisms:

  • Mapping as a direct access devices allows the guest to directly access the host memory pages (such as via Execute In Place (XIP)), bypassing the guest page cache. This provides both time and space optimizations.
  • Mapping as a direct access device inside the VM allows pages from the host to be demand loaded using page faults, rather than having to make requests via a virtualized device (causing expensive VM exits/hypercalls), thus providing a speed optimization.
  • Utilizing MAP_SHARED shared memory on the host allows the host to efficiently share pages.

Kata Containers uses the following steps to set up the DAX mappings:

  1. QEMU is configured with an NVDIMM memory device, with a memory file backend to map in the host-side file into the virtual NVDIMM space.
  2. The guest kernel command line mounts this NVDIMM device with the DAX feature enabled, allowing direct page mapping and access, thus bypassing the guest page cache.

DAX

Information on the use of NVDIMM via QEMU is available in the QEMU source code