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vfkit Command Line

The vfkit executable can be used to create a virtual machine (VM) using macOS virtualization framework. The virtual machine will be terminated as soon as the vfkit process exits. Its configuration can be specified through command line options.

Specifying VM bootloader configuration is mandatory. Device configuration is optional, but most VM will need a disk image and a network interface to be configured.

Generic Options

  • --log-level

Set the log-level for VFKit. Supported values are debug, info, and error.

  • --restful-uri

The URI (address) of the RESTful service. The default is tcp://localhost:8081. Valid schemes are tcp, none, or unix. In the case of unix, the "host" portion would be a path to where the unix domain socket will be stored. A scheme of none disables the RESTful service.

Virtual Machine Resources

These options specify the amount of RAM and the number of CPUs which will be available to the virtual machine. They are mandatory.

  • --cpus

Number of virtual CPUs (vCPU) available in the VM. It defaults to 1 vCPU.

  • --memory

Amount of memory available in the virtual machine. The value is in MiB (mebibytes, 1024 * 1024 bytes), and the default is 512 MiB.

Time Synchronization Configuration

Description

When the host system is suspended, the guest clock stops running, and it's unable to get back to the correct time upon resume. The --timesync option can be used to let vfkit set the guest clock to the correct time when it detects the host. At the moment, this can only be done using qemu-guest-agent, which has to be installed in the guest. It must be configured to communicate over virtio-vsock.

Arguments

  • vsockPort: vsock port used for communication with the guest agent.

Bootloader Configuration

A bootloader is required to tell vfkit how it should start the guest OS.

Linux bootloader

Description

--bootloader linux replaces the legacy --kernel, --kernel-cmdline and --initrd options. It allows to specify which kernel and initrd should be used when starting the VM.

Arguments

  • kernel: path to the kernel to use to start the virtual machine. The kernel must be uncompressed or the VM will hang when trying to start. See the kernel documentation for more details.
  • initrd: path to the initrd file to use when starting the virtual machine.
  • cmdline: kernel command line to use when starting the virtual machine.

Example

--bootloader linux,kernel=~/kernels/vmlinuz-5.18.18-200.fc36.aarch64,initrd=~/kernels/initramfs-5.18.18-200.fc36.aarch64.img,cmdline="\"console=hvc0 root=UUID=164b4fc3-dc5a-40ea-a40b-c689a7bf41cf rw\""

The kernel command line must be enclosed in ", and depending on your shell, they might need to be escaped (\")

macOS bootloader

Description

--bootloader macos is required to run macOS VMs. You must use an arm64/Apple silicon device running macOS 12 or later. Due to hardcoded limitations in the Apple Virtualization framework, it's not possible to run more than two macOS VMs at a time. Since macOS guests can't run headlessly, you'll need to enable a GUI, even if you only plan to interact with the VM over SSH.

Arguments

  • machineIdentifierPath: absolute path to a binary property list containing a unique ECID identifier for the VM
  • hardwareModelPath: absolute path to a binary property list defining OS version support
  • auxImagePath: absolute path to the auxiliary storage file with NVRAM contents and the iBoot bootloader

Example

--bootloader macos,machineIdentifierPath=/Users/virtuser/VM.bundle/MachineIdentifier,hardwareModelPath=/Users/virtuser/VM.bundle/HardwareModel,auxImagePath=/Users/virtuser/VM.bundle/AuxiliaryStorage

EFI bootloader

Description

--bootloader efi is only available when running on macOS 13 or newer. This allows to boot a disk image using EFI, which removes the need for providing external kernel/initrd/... The disk image bootloader will be started by the EFI firmware, which will in turn know which kernel it should be booting.

Arguments

  • variable-store: path to a file which EFI can use to store its variables
  • create: indicate whether the variable-store file should be created or not if missing.

Example

--bootloader efi,variable-store=/Users/virtuser/efi-variable-store,create

Deprecated options

Description

The --kernel, --initrd and --kernel-cmdline options are deprecated and have been replaced by the more generic --bootloader option.

Options

  • --kernel

Path to the kernel to use to start the virtual machine. The kernel must be uncompressed or the VM will hang when trying to start. See the kernel documentation for more details.

  • --initrd

Path to the initrd file to use when starting the virtual machine.

  • --kernel-cmdline

Kernel command line to use when starting the virtual machine.

Device Configuration

Various devices can be added to the virtual machines. They are all paravirtualized devices using VirtIO. They are grouped under the --device command line flag.

Disk

Description

The --device virtio-blk option adds a disk to the virtual machine. The disk is backed by an image file on the host machine. This file is a raw image file. See also vz/CreateDiskImage.

Thin images

Apple Virtualization Framework only supports raw disk images and ISO images. There is no support for thin image formats such as qcow2.

However, APFS, the default macOS filesystem has support for sparse files and copy-on-write files, so it offers the main features of thin image formats.

A sparse raw image can be created/expanded using the truncate command or using truncate(2). For example, an empty 1GiB disk can be created with truncate -s 1G vfkit.img. Such an image will only use disk space when content is written to it. It initially only uses a few bytes of actual disk space even if its size is 1G.

A copy-on-write image is a raw image file which references a backing file. Its initial content is the same as its backing file, and the data is shared with the backing file. This means the copy-on-write image does not use extra disk space when it's created. When this image is modified, the changes will only be made to the copy-on-write image, and not to the backing file. Only the modified data will use actual disk space. A copy-on-write image can be created using cp -c or clonefile(2).

Arguments

  • path: the absolute path to the disk image file.
  • deviceId: /dev/disk/by-id/ identifier to use for this device.

Example

This adds a virtio-blk device to the VM which will be backed by the raw image at /Users/virtuser/vfkit.img:

--device virtio-blk,path=/Users/virtuser/vfkit.img

NVM Express

Description

The --device nvme option adds a NVMe device to the virtual machine. The disk is backed by an image file on the host machine. This file is a raw image file.

Arguments

  • path: the absolute path to the disk image file.

Example

This adds a NVMe device to the VM which will be backed by the disk image at /Users/virtuser/image.img:

--device nvme,path=/Users/virtuser/image.img

USB Mass Storage

Description

The --device usb-mass-storage option adds a USB mass storage device to the virtual machine. The disk is backed by an image file on the host machine. This file is a raw image file or an ISO image.

Arguments

  • path: the absolute path to the disk image file.
  • readonly: if specified the device will be read only.

Example

This adds a read only USB mass storage device to the VM which will be backed by the ISO image at /Users/virtuser/distro.iso:

--device usb-mass-storage,path=/Users/virtuser/distro.iso,readonly

Networking

Description

The --device virtio-net option adds a network interface to the virtual machine. If it gets its IP address through DHCP, its IP can be found in /var/db/dhcpd_leases on the host.

Arguments

  • mac: optional argument to specify the MAC address of the VM. If it's omitted, a random MAC address will be used.
  • fd: file descriptor to attach to the guest network interface. The file descriptor must be a connected datagram socket. See VZFileHandleNetworkDeviceAttachment for more details.
  • nat: guest network traffic will be NAT'ed through the host. This is the default. See VZNATNetworkDeviceAttachment for more details.
  • unixSocketPath: path to a unix socket to attach to the guest network interface. See VZFileHandleNetworkDeviceAttachment for more details.

fd, nat, unixSocketPath are mutually exclusive.

Example

This adds a virtio-net device to the VM with 52:54:00:70:2b:71 as its MAC address:

--device virtio-net,nat,mac=52:54:00:70:2b:71

This adds a virtio-net device to the VM, and redirects all the network traffic on the corresponding guest network interface to /Users/virtuser/virtio-net.sock:

--device virtio-net,unixSocketPath=/Users/virtuser/virtio-net.sock

This is useful in combination with usermode networking stacks such as gvisor-tap-vsock.

Serial Port

Description

The --device virtio-serial option adds a serial device to the virtual machine. This is useful to redirect text output from the virtual machine to a log file. The logFilePath and stdio arguments are mutually exclusive.

Arguments

  • logFilePath: path where the serial port output should be written.
  • stdio: uses stdin/stdout for the serial console input/output.

Example

This adds a virtio-serial device to the VM, and will log everything which is written to this device to /Users/virtuser/vfkit.log:

--device virtio-serial,logFilePath=/Users/virtuser/vfkit.log

This adds a virtio-serial device to the VM, and the terminal vfkit is launched from will be used as an interactive serial console for that device:

--device virtio-serial,stdio

Random Number Generator

Description

The --device virtio-rng option adds a random number generator device to the virtual machine. It will feed entropy from the host to the virtual machine, as VMs often do not have many entropy sources.

Example

This adds a virtio-rng device to the VM:

--device virtio-rng

virtio-vsock communication

Description

The --device virtio-vsock option adds a virtio-vsock communication channel between the host and the guest See man 4 vsock for more details. macOS does not have host support for AF_VSOCK sockets so the vsock port will be exposed as a unix socket on the host.

--device virtio-vsock can be specified multiple times on the command line to allow communication over multiple vsock ports. There will only be a single virtio-vsock device added to the VM regardless of the number of --device virtio-vsock occurrences on the command line.

Arguments

  • port: vsock port to use for the VM/host communication.
  • socketURL: path to the unix socket to use on the host for the vsock communication.
  • connect: indicates that the host will connect to the guest over vsock.
  • listen : indicates that the host will be listening for vsock connections (default).

Example

This allows virtio-vsock communication from the guest to the host over vsock port 5:

--device virtio-vsock,port=5,socketURL=/Users/virtuser/vfkit-5.sock

The socket can be created on the host with nc -U -l /Users/virtuser/vfkit-5.sock, and the guest can connect to it with nc --vsock 2 5.

This allows virtio-vsock communication from the host to the guest over vsock port 6:

--device virtio-vsock,port=6,socketURL=/Users/virtuser/vfkit-6.sock,connect

The socket can be created on the guest with nc --vsock --listen 3 6, and the host can connect to it with nc -U /Users/virtuser/vfkit-6.sock,connect.

File Sharing

Description

The -device virtio-fs option allows to share directories between the host and the guest. The sharing will be done using virtio-fs. The share can be mounted in the guest with mount -t virtiofs vfkitTag /mnt, with vfkitTag corresponding to the value of the mountTag option.

Arguments

  • sharedDir: absolute path to the host directory to share with the guest.
  • mountTag: tag which will be used to mount the shared directory in the guest.

Example

This will share /Users/virtuser/vfkit with the guest:

--device virtio-fs,sharedDir=/Users/virtuser/vfkit/,mountTag=vfkit-share

The share can then be mounted in Linux guests with:

mount -t virtiofs vfkit-share /mount

and on macOS with:

mkdir /tmp/tag && mount_virtiofs vfkit-share /tmp/tag

Rosetta

Description

The -device rosetta option allows to use Rosetta to run x86_64 binaries in an arm64 linux VM. This option will share a directory containing the rosetta binaries over virtio-fs. The share can be mounted in the guest with mount -t virtiofs vfkitTag /mnt, with vfkitTag corresponding to the value of the mountTag option. Then, binfmt needs to be configured to use this rosetta binary for x86_64 executables. On systems using systemd, this can be achieved by creating a /etc/binfmt.d/rosetta.conf file with this content (/mnt/rosetta is the full path to the rosetta binary):

:rosetta:M::\x7fELF\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x3e\x00:\xff\xff\xff\xff\xff\xfe\xfe\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/mnt/rosetta:F

and then running systemctl restart systemd-binfmt.

This option is only available on machine with Apple CPUs, vfkit will fail with an error if it's used on Intel machines.

See https://developer.apple.com/documentation/virtualization/running_intel_binaries_in_linux_vms_with_rosetta?language=objc for more details.

Arguments

  • mountTag: tag which will be used to mount the rosetta share in the guest.
  • install: indicates to automatically install rosetta on systems where it's missing. By default, an error will be reported if --device rosetta is used when rosetta is not installed.

Example

This adds rosetta support to the guest:

--device rosetta,mountTag=rosetta-share

The share can then be mounted with mount -t virtiofs rosetta-share /mnt.

GPU

Description

The --device virtio-gpu option allows the user to add graphical devices to the virtual machine.

Arguments

  • width: the horizontal resolution of the graphical device's resolution. Defaults to 800
  • height: the vertical resolution of the graphical device's resolution. Defaults to 600

Example

--device virtio-gpu,width=1920,height=1080

Input

Description

The --device virtio-input option allows the user to add an input device to the virtual machine. This currently supports pointing and keyboard devices.

Arguments

None

Example

--device virtio-input,pointing

RESTful API

To interact with the RESTful API, append a valid scheme to your base command: --restful-uri tcp://localhost:8081.

Get VM state

Obtain the state of the virtual machine that is being run by vfkit.

Request:

GET /vm/state

Response: { "state": string, "canStart": bool, "canPause": bool, "canResume": bool, "canStop": bool, "canHardStop": bool }

canHardStop is only supported on macOS 12 and newer, false will always be returned on older versions.

Change VM State

Change the state of the virtual machine. Valid state values are:

  • HardStop
  • Pause
  • Resume
  • Stop
POST /vm/state { "state": "new value"}

Response: HTTP 200

Inspect VM

Get description of the virtual machine

GET /vm/inspect

Response: { "cpus": uint, "memory": uint64, "devices": []config.VirtIODevice }

Enabling a Graphical User Interface

Add a virtio-gpu device

In order to successfully start a graphical application window, a virtio-gpu device must be added to the virtual machine.

Pass the --gui flag

In order to tell vfkit that you want to start a graphical application window, you need to pass the --gui flag in your command.

Usage

Proper use of this flag may look similar to the following section of a command:

--device virtio-input,keyboard --device virtio-input,pointing --device virtio-gpu,width=1920,height=1080 --gui