• Cloud Hypervisor API
  • External API
    • REST API
      • REST API Location and availability
      • REST API Endpoints
        • Virtual Machine Manager (VMM) Actions
        • Virtual Machine (VM) Actions
      • REST API Examples
        • Create a Virtual Machine
        • Boot a Virtual Machine
        • Dump Virtual Machine Information
        • Reboot a Virtual Machine
        • Shut a Virtual Machine Down
    • D-Bus API
      • D-Bus API Location and availability
      • D-Bus API Interface
    • Command Line Interface
    • REST API, D-Bus API and CLI Architectural Relationship
  • Internal API
    • Goals and Design
  • End to End Example

Cloud Hypervisor API

The Cloud Hypervisor API is made of 2 distinct interfaces:

1. The External API This is the user facing API. Users and operators can control and manage the Cloud Hypervisor through various options including a REST API, a Command Line Interface (CLI) or a D-Bus based API, which is not compiled into Cloud Hypervisor by default.

2. The internal API, based on rust's Multi-Producer, Single-Consumer (MPSC) module. This API is used internally by the Cloud Hypervisor threads to communicate with each other.

The goal of this document is to describe the Cloud Hypervisor API as a whole, and to outline how the internal and external APIs are architecturally related.

External API

REST API

The Cloud Hypervisor REST API triggers VM and VMM specific actions, and as such it is designed as a collection of RPC-style, static methods.

The API is OpenAPI 3.0 compliant. Please consult the Cloud Hypervisor OpenAPI Document for more details about the API payloads and responses.

REST API Location and availability

The REST API, if enabled, is available as soon as the Cloud Hypervisor binary is started, through either a local UNIX socket as given in the Cloud Hypervisor option --api-socket path=... or a fd with --api-socket fd=....

$ ./target/debug/cloud-hypervisor --api-socket path=/tmp/cloud-hypervisor.sock

REST API Endpoints

The Cloud Hypervisor API exposes the following actions through its endpoints:

Virtual Machine Manager (VMM) Actions

Virtual Machine (VM) Actions

• The vmcoredump action is available exclusively for the x86_64 architecture and can be executed only when the guest_debug feature is enabled. Without this feature, the corresponding REST API or D-Bus API endpoints are not available.

REST API Examples

For the following set of examples, we assume Cloud Hypervisor is started with the REST API available at /tmp/cloud-hypervisor.sock:

$ ./target/debug/cloud-hypervisor --api-socket /tmp/cloud-hypervisor.sock

Create a Virtual Machine

We want to create a virtual machine with the following characteristics:

• 4 vCPUs
• 1 GB of RAM
• 1 virtio based networking interface
• Direct kernel boot from a custom 5.6.0-rc4 Linux kernel located at /opt/clh/kernel/vmlinux-virtio-fs-virtio-iommu
• Using a Ubuntu image as its root filesystem, located at /opt/clh/images/focal-server-cloudimg-amd64.raw

#!/usr/bin/env bash

curl --unix-socket /tmp/cloud-hypervisor.sock -i \
     -X PUT 'http://localhost/api/v1/vm.create'  \
     -H 'Accept: application/json'               \
     -H 'Content-Type: application/json'         \
     -d '{
         "cpus":{"boot_vcpus": 4, "max_vcpus": 4},
         "payload":{"kernel":"/opt/clh/kernel/vmlinux-virtio-fs-virtio-iommu", "cmdline":"console=ttyS0 console=hvc0 root=/dev/vda1 rw"},
         "disks":[{"path":"/opt/clh/images/focal-server-cloudimg-amd64.raw"}],
         "rng":{"src":"/dev/urandom"},
         "net":[{"ip":"192.168.10.10", "mask":"255.255.255.0", "mac":"12:34:56:78:90:01"}]
         }'

Boot a Virtual Machine

Once the VM is created, we can boot it:

#!/usr/bin/env bash

curl --unix-socket /tmp/cloud-hypervisor.sock -i -X PUT 'http://localhost/api/v1/vm.boot'

Dump Virtual Machine Information

We can fetch information about any VM as soon as it's created:

#!/usr/bin/env bash

curl --unix-socket /tmp/cloud-hypervisor.sock -i \
     -X GET 'http://localhost/api/v1/vm.info' \
     -H 'Accept: application/json'

Reboot a Virtual Machine

We can reboot a VM that's already booted:

#!/usr/bin/env bash

curl --unix-socket /tmp/cloud-hypervisor.sock -i -X PUT 'http://localhost/api/v1/vm.reboot'

Shut a Virtual Machine Down

Once booted, we can shut a VM down from the REST API:

#!/usr/bin/env bash

curl --unix-socket /tmp/cloud-hypervisor.sock -i -X PUT 'http://localhost/api/v1/vm.shutdown'

D-Bus API

Cloud Hypervisor offers a D-Bus API as an alternative to its REST API. This D-Bus API fully reflects the functionality of the REST API, exposing the same group of endpoints. It can be a drop-in replacement since it also consumes/produces JSON.

In addition, the D-Bus API also exposes events from event-monitor in the form of a D-Bus signal to which users can subscribe. For more information, see D-Bus API Interface.

D-Bus API Location and availability

This feature is not compiled into Cloud Hypervisor by default. Users who wish to use the D-Bus API must explicitly enable it with the dbus_api feature flag when compiling Cloud Hypervisor.

$ ./scripts/dev_cli.sh build --release --libc musl -- --features dbus_api

Once this feature is enabled, it can be configured with the following CLI options:

  --dbus-service-name
                    well known name of the service
  --dbus-object-path
                    object path to serve the dbus interface
  --dbus-system-bus use the system bus instead of a session bus

Example invocation:

$ ./cloud-hypervisor --dbus-service-name "org.cloudhypervisor.DBusApi" \
                     --dbus-object-path "/org/cloudhypervisor/DBusApi"

This will start serving a service with the name org.cloudhypervisor.DBusApi1 which in turn can be used to control and manage Cloud Hypervisor.

D-Bus API Interface

Please refer to the REST API documentation for everything that is in common with the REST API. As previously mentioned, the D-Bus API can be used as a drop-in replacement for the REST API.

The D-Bus interface also exposes a signal, named Event, which is emitted whenever a new event is published from the event-monitor crate. Here is its definition in XML format:

<node>
  <interface name="org.cloudhypervisor.DBusApi1">
    <signal name="Event">
      <arg name="event" type="s"/>
    </signal>
  </interface>
</node>

Command Line Interface

The Cloud Hypervisor Command Line Interface (CLI) can only be used for launching the Cloud Hypervisor binary, i.e. it cannot be used for controlling the VMM or the launched VM once they're up and running.

If you want to inspect the VMM, or control the VM after launching Cloud Hypervisor from the CLI, you must use either the REST API or the D-Bus API.

From the CLI, one can:

1. Create and boot a complete virtual machine by using the CLI options to build the VM config. Run cloud-hypervisor --help for a complete list of CLI options. As soon as the cloud-hypervisor binary is launched, contrary to the D-Bus API, the REST API is available for controlling and managing the VM. The D-Bus API doesn't start automatically and needs to be explicitly configured in order to be run.
2. Start either the REST API, D-Bus API or both simultaneously without passing any VM configuration options. The VM can then be asynchronously created and booted by calling API methods of choice. It should be noted that one external API does not exclude another; it is possible to have both the REST and D-Bus APIs running simultaneously.

REST API, D-Bus API and CLI Architectural Relationship

The REST API, D-Bus API and the CLI all rely on a common, internal API.

The CLI options are parsed by the clap crate and then translated into internal API commands.

The REST API is processed by an HTTP thread using the Firecracker's micro_http crate. As with the CLI, the HTTP requests eventually get translated into internal API commands.

The D-Bus API is implemented using the zbus crate and runs in its own thread. Whenever it needs to call the internal API, the blocking crate is used to perform the call in zbus' async context.

As a summary, the REST API, the D-Bus API and the CLI are essentially frontends for the internal API:

                                  +------------------+
                        REST API  |                  |
                       +--------->+    micro_http    +--------+
                       |          |                  |        |
                       |          +------------------+        |
                       |                                      |      +------------------------+
                       |                                      |      |                        |
+------------+         |            +----------+              |      |                        |
|            |         |  D-Bus API |          |              |      | +--------------+       |
|    User    +---------+----------->+   zbus   +--------------+------> | Internal API |       |
|            |         |            |          |              |      | +--------------+       |
+------------+         |            +----------+              |      |                        |
                       |                                      |      |                        |
                       |                                      |      +------------------------+
                       |            +----------+              |                 VMM
                       |     CLI    |          |              |
                       +----------->+   clap   +--------------+
                                    |          |
                                    +----------+

Internal API

The Cloud Hypervisor internal API, as its name suggests, is used internally by the different Cloud Hypervisor threads (VMM, HTTP, D-Bus, control loop, etc) to send commands and responses to each other.

It is based on rust's Multi-Producer, Single-Consumer (MPSC), and the single consumer (a.k.a. the API receiver) is the Cloud Hypervisor control loop.

API producers are the HTTP thread handling the REST API, the D-Bus thread handling the D-Bus API and the main thread that initially parses the CLI.

Goals and Design

The internal API is designed for controlling, managing and inspecting a Cloud Hypervisor VMM and its guest. It is a backend for handling external, user visible requests through the REST API, the D-Bus API or the CLI interfaces.

The API follows a command-response scheme that closely maps the REST API. Any command must be replied to with a response.

Commands are MPSC based messages and are received and processed by the VMM control loop.

In order for the VMM control loop to respond to any internal API command, it must be able to send a response back to the MPSC sender. For that purpose, all internal API command payload carry the Sender end of an MPSC channel.

The sender of any internal API command is therefore responsible for:

1. Creating an MPSC response channel.
2. Passing the Sender end of the response channel as part of the internal API command payload.
3. Waiting for the internal API command's response on the Receiver end of the response channel.

End to End Example

In order to further understand how the external and internal Cloud Hypervisor APIs work together, let's look at a complete VM creation flow, from the REST API call, to the reply the external user will receive:

1. A user or operator sends an HTTP request to the Cloud Hypervisor REST API in order to create a virtual machine:
   shellCopy

   #!/usr/bin/env bash
   
    curl --unix-socket /tmp/cloud-hypervisor.sock -i \
    	-X PUT 'http://localhost/api/v1/vm.create'  \
    	-H 'Accept: application/json'               \
    	-H 'Content-Type: application/json'         \
    	-d '{
    		"cpus":{"boot_vcpus": 4, "max_vcpus": 4},
    		"payload":{"kernel":"/opt/clh/kernel/vmlinux-virtio-fs-virtio-iommu", "cmdline":"console=ttyS0 console=hvc0 root=/dev/vda1 rw"},
    		"disks":[{"path":"/opt/clh/images/focal-server-cloudimg-amd64.raw"}],
    		"rng":{"src":"/dev/urandom"},
    		"net":[{"ip":"192.168.10.10", "mask":"255.255.255.0", "mac":"12:34:56:78:90:01"}]
    		}'
   

2. The Cloud Hypervisor HTTP thread processes the request and de-serializes the HTTP request JSON body into an internal VmConfig structure.
3. The Cloud Hypervisor HTTP thread creates an MPSC channel for the internal API server to send its response back.
4. The Cloud Hypervisor HTTP thread prepares an internal API command for creating a virtual machine. The command's payload is made of the de-serialized VmConfig structure and the response channel:
   RustCopy

   VmCreate(Arc<Mutex<VmConfig>>, Sender<ApiResponse>)
   

5. The Cloud Hypervisor HTTP thread sends the internal API command, and waits for the response:
   RustCopy

   // Send the VM creation request.
    api_sender
        .send(ApiRequest::VmCreate(config, response_sender))
        .map_err(ApiError::RequestSend)?;
    api_evt.write(1).map_err(ApiError::EventFdWrite)?;
   
    response_receiver.recv().map_err(ApiError::ResponseRecv)??;
   

6. The Cloud Hypervisor control loop receives the command, as it listens on the internal API MPSC channel:
   RustCopy

   // Read from the API receiver channel
   let api_request = api_receiver.recv().map_err(Error::ApiRequestRecv)?;
   

7. The Cloud Hypervisor control loop matches the received internal API against the VmCreate payload, and extracts both the VmConfig structure and the Sender from the command payload. It stores the VmConfig structure and replies back to the sender (The HTTP thread):
   RustCopy

   match api_request {
       ApiRequest::VmCreate(config, sender) => {
    	   // We only store the passed VM config.
    	   // The VM will be created when being asked to boot it.
    	   let response = if self.vm_config.is_none() {
    		   self.vm_config = Some(config);
    		   Ok(ApiResponsePayload::Empty)
    	   } else {
    		   Err(ApiError::VmAlreadyCreated)
    	   };
   
           sender.send(response).map_err(Error::ApiResponseSend)?;
       }
   

8. The Cloud Hypervisor HTTP thread receives the internal API command response as the return value from its VmCreate HTTP handler. Depending on the control loop internal API response, it generates the appropriate HTTP response:
   RustCopy

   // Call vm_create()
   match vm_create(api_notifier, api_sender, Arc::new(Mutex::new(vm_config)))
       .map_err(HttpError::VmCreate)
   {
       Ok(_) => Response::new(Version::Http11, StatusCode::NoContent),
       Err(e) => error_response(e, StatusCode::InternalServerError),
   }
   

9. The Cloud Hypervisor HTTP thread sends the formed HTTP response back to the user. This is abstracted by the micro_http crate.