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cloud-hypervisor

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A Virtual Machine Monitor for modern Cloud workloads. Features include CPU, memory and device hotplug, support for running Windows and Linux guests, device offl

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A Virtual Machine Monitor for modern Cloud workloads. Features include CPU, memory and device hotplug, support for running Windows and Linux guests, device offl

  • 1. What is Cloud Hypervisor?
    • Objectives
      • High Level
      • Architectures
      • Guest OS
  • 2. Getting Started
    • Host OS
    • Use Pre-built Binaries
    • Packages
    • Building from Source
    • Booting Linux
      • Firmware Booting
      • Custom Kernel and Disk Image
        • Building your Kernel
        • Disk image
        • Booting the guest VM
  • 3. Status
    • Hot Plug
    • Device Model
    • Roadmap
  • 4. Relationship with Rust VMM Project
    • Differences with Firecracker and crosvm
  • 5. Community
    • Contribute
    • Slack
    • Mailing list
    • Security issues

1. What is Cloud Hypervisor?

Cloud Hypervisor is an open source Virtual Machine Monitor (VMM) that runs on top of the KVM hypervisor and the Microsoft Hypervisor (MSHV).

The project focuses on running modern, Cloud Workloads, on specific, common, hardware architectures. In this case Cloud Workloads refers to those that are run by customers inside a Cloud Service Provider. This means modern operating systems with most I/O handled by paravirtualised devices (e.g. virtio), no requirement for legacy devices, and 64-bit CPUs.

Cloud Hypervisor is implemented in Rust and is based on the Rust VMM crates.

Objectives

High Level

  • Runs on KVM or MSHV
  • Minimal emulation
  • Low latency
  • Low memory footprint
  • Low complexity
  • High performance
  • Small attack surface
  • 64-bit support only
  • CPU, memory, PCI hotplug
  • Machine to machine migration

Architectures

Cloud Hypervisor's main supported architectures are x86-64 and AArch64, with functionality varying across these platforms. The functionality differences between x86-64 and AArch64 are documented in #1125. The riscv64 architecture support is experimental and offers limited functionality. For more details and instructions, please refer to riscv documentation.

Guest OS

Cloud Hypervisor supports 64-bit Linux and Windows 10/Windows Server 2019.

2. Getting Started

The following sections describe how to build and run Cloud Hypervisor.

Prerequisites for AArch64

  • AArch64 servers (recommended) or development boards equipped with the GICv3 interrupt controller.

Host OS

For required KVM functionality and adequate performance the recommended host kernel version is 5.13. The majority of the CI currently tests with kernel version 5.15.

Use Pre-built Binaries

The recommended approach to getting started with Cloud Hypervisor is by using a pre-built binary. Binaries are available for the latest release. Use cloud-hypervisor-static for x86-64 or cloud-hypervisor-static-aarch64 for AArch64 platform.

Packages

For convenience, packages are also available targeting some popular Linux distributions. This is thanks to the Open Build Service. The OBS README explains how to enable the repository in a supported Linux distribution and install Cloud Hypervisor and accompanying packages. Please report any packaging issues in the obs-packaging repository.

Building from Source

Please see the instructions for building from source if you do not wish to use the pre-built binaries.

Booting Linux

Cloud Hypervisor boots guests in one of two ways. The first is direct kernel boot, where a kernel image is passed to --kernel. The x86-64 kernel must be built with PVH support or be a bzImage. The second is firmware boot, where a firmware image is passed to --firmware and brings up the guest's normal boot loader.

Two firmware options are supported, and which one works best depends on the guest OS. Rust Hypervisor Firmware is a lightweight Rust-based PVH firmware. The edk2 UEFI firmware is called CLOUDHV.fd for x86-64 and CLOUDHV_EFI.fd for AArch64. Prebuilt binaries for both are available at their respective releases pages, Rust Hypervisor Firmware and our edk2 fork. The edk2 fork carries customizations required to boot AArch64 guests on cloud-hypervisor. See docs/uefi.md for differences with upstream tianocore/edk2.

Firmware Booting

Cloud Hypervisor supports booting disk images containing all needed components to run cloud workloads, a.k.a. cloud images.

The following sample commands will download an Ubuntu Cloud image, converting it into a format that Cloud Hypervisor can use and a firmware to boot the image with.

$ wget https://cloud-images.ubuntu.com/focal/current/focal-server-cloudimg-amd64.img
$ qemu-img convert -p -f qcow2 -O raw focal-server-cloudimg-amd64.img focal-server-cloudimg-amd64.raw
$ wget https://github.com/cloud-hypervisor/rust-hypervisor-firmware/releases/download/0.4.2/hypervisor-fw

The Ubuntu cloud images do not ship with a default password so it necessary to use a cloud-init disk image to customise the image on the first boot. A basic cloud-init image is generated by this script. This seeds the image with a default username/password of cloud/cloud123. It is only necessary to add this disk image on the first boot. Script also assigns default IP address using test_data/cloud-init/ubuntu/local/network-config details with --net "mac=12:34:56:78:90:ab,tap=" option. Then the matching mac address interface will be enabled as per network-config details.

$ sudo setcap cap_net_admin+ep ./cloud-hypervisor
$ ./create-cloud-init.sh
$ ./cloud-hypervisor \
	--firmware ./hypervisor-fw \
	--disk path=focal-server-cloudimg-amd64.raw,image_type=raw \
		path=/tmp/ubuntu-cloudinit.img,image_type=raw \
	--cpus boot=4 \
	--memory size=1024M \
	--net "tap=,mac=,ip=,mask="

If access to the firmware messages or interaction with the boot loader (e.g. GRUB) is required then it necessary to switch to the serial console instead of virtio-console.

$ ./cloud-hypervisor \
	--kernel ./hypervisor-fw \
	--disk path=focal-server-cloudimg-amd64.raw,image_type=raw \
		path=/tmp/ubuntu-cloudinit.img,image_type=raw \
	--cpus boot=4 \
	--memory size=1024M \
	--net "tap=,mac=,ip=,mask=" \
	--serial tty \
	--console off

Booting: --firmware vs --kernel

The following scenarios are supported by Cloud Hypervisor to bootstrap a VM, i.e., to load a payload/bootitem(s):

  • Provide firmware
  • Provide kernel [+ cmdline]\ [+ initrd]

Please note that our Cloud Hypervisor firmware (hypervisor-fw) has a Xen PVH boot entry, therefore it can also be booted via the --kernel parameter, as seen in some examples.

Custom Kernel and Disk Image

Building your Kernel

Cloud Hypervisor also supports direct kernel boot. For x86-64, a vmlinux ELF kernel (compiled with PVH support) or a regular bzImage are supported. In order to support development there is a custom branch; however provided the required options are enabled any recent kernel will suffice.

To build the kernel:

# Clone the Cloud Hypervisor Linux branch
$ git clone --depth 1 https://github.com/cloud-hypervisor/linux.git -b ch-6.16.9 linux-cloud-hypervisor
$ pushd linux-cloud-hypervisor
$ make ch_defconfig
# Do native build of the x86-64 kernel
$ KCFLAGS="-Wa,-mx86-used-note=no" make bzImage -j `nproc`
# Do native build of the AArch64 kernel
$ make -j `nproc`
$ popd

For x86-64, the vmlinux kernel image will then be located at linux-cloud-hypervisor/arch/x86/boot/compressed/vmlinux.bin. For AArch64, the Image kernel image will then be located at linux-cloud-hypervisor/arch/arm64/boot/Image.

Disk image

For the disk image the same Ubuntu image as before can be used. This contains an ext4 root filesystem.

$ wget https://cloud-images.ubuntu.com/focal/current/focal-server-cloudimg-amd64.img # x86-64
$ wget https://cloud-images.ubuntu.com/focal/current/focal-server-cloudimg-arm64.img # AArch64
$ qemu-img convert -p -f qcow2 -O raw focal-server-cloudimg-amd64.img focal-server-cloudimg-amd64.raw # x86-64
$ qemu-img convert -p -f qcow2 -O raw focal-server-cloudimg-arm64.img focal-server-cloudimg-arm64.raw # AArch64

Booting the guest VM

These sample commands boot the disk image using the custom kernel whilst also supplying the desired kernel command line.

  • x86-64
$ sudo setcap cap_net_admin+ep ./cloud-hypervisor
$ ./create-cloud-init.sh
$ ./cloud-hypervisor \
	--kernel ./linux-cloud-hypervisor/arch/x86/boot/compressed/vmlinux.bin \
	--disk path=focal-server-cloudimg-amd64.raw,image_type=raw \
		path=/tmp/ubuntu-cloudinit.img,image_type=raw \
	--cmdline "console=hvc0 root=/dev/vda1 rw" \
	--cpus boot=4 \
	--memory size=1024M \
	--net "tap=,mac=,ip=,mask="
  • AArch64
$ sudo setcap cap_net_admin+ep ./cloud-hypervisor
$ ./create-cloud-init.sh
$ ./cloud-hypervisor \
	--kernel ./linux-cloud-hypervisor/arch/arm64/boot/Image \
	--disk path=focal-server-cloudimg-arm64.raw,image_type=raw \
		path=/tmp/ubuntu-cloudinit.img,image_type=raw \
	--cmdline "console=hvc0 root=/dev/vda1 rw" \
	--cpus boot=4 \
	--memory size=1024M \
	--net "tap=,mac=,ip=,mask="

If earlier kernel messages are required the serial console should be used instead of virtio-console.

  • x86-64
$ ./cloud-hypervisor \
	--kernel ./linux-cloud-hypervisor/arch/x86/boot/compressed/vmlinux.bin \
	--console off \
	--serial tty \
	--disk path=focal-server-cloudimg-amd64.raw,image_type=raw \
	--cmdline "console=ttyS0 root=/dev/vda1 rw" \
	--cpus boot=4 \
	--memory size=1024M \
	--net "tap=,mac=,ip=,mask="
  • AArch64
$ ./cloud-hypervisor \
	--kernel ./linux-cloud-hypervisor/arch/arm64/boot/Image \
	--console off \
	--serial tty \
	--disk path=focal-server-cloudimg-arm64.raw,image_type=raw \
	--cmdline "console=ttyAMA0 root=/dev/vda1 rw" \
	--cpus boot=4 \
	--memory size=1024M \
	--net "tap=,mac=,ip=,mask="

3. Status

Cloud Hypervisor is under active development. The following stability guarantees are currently made:

  • The API (including command line options) will not be removed or changed in a breaking way without a minimum of 2 major releases notice. Where possible warnings will be given about the use of deprecated functionality and the deprecations will be documented in the release notes.

  • Point releases will be made between individual releases where there are substantial bug fixes or security issues that need to be fixed. These point releases will only include bug fixes.

Currently the following items are not guaranteed across updates:

  • Snapshot/restore is not supported across different versions
  • Live migration is not supported across different versions
  • The following features are considered experimental and may change substantially between releases: TDX, vfio-user, vDPA.

Further details can be found in the release documentation.

As of 2023-01-03, the following cloud images are supported:

  • Ubuntu Focal (focal-server-cloudimg-{amd64,arm64}.img)
  • [Ubunt

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PublishedAug 1, 2026
UpdatedSep 17, 2026
Category编程语言
PricingOpen source

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