Guest OS and bootloader of a virtual machine

The osType parameter defines the operating system type and applies the optimal set of virtual devices and parameters for a virtual machine (VM) to work correctly.

Supported values:

• Generic (default): For Linux and other operating systems. The standard virtual device configuration is used.
• Windows: For Microsoft Windows operating systems. Automatically enables Hyper-V features, a TPM device, and other settings optimized for Windows.
• Legacy: For operating systems without built-in AHCI and virtio drivers: Windows XP, Windows 2000, Windows Server 2003, DOS-era systems, and Linux with a kernel older than 2.6.19. Such a VM gets the i440fx chipset, and with enableParavirtualization: false it also gets the IDE bus for disks and CD-ROM and the RTL8139 network adapter, whose drivers these operating systems have.

The set of virtual devices the guest OS sees:

USB device passthrough works for Legacy, but the UHCI controller is limited to the USB 1.1 speed of 12 Mbps, so fast storage in such a VM hits the bus limit.

Choose Legacy when the guest operating system can’t work with an AHCI controller, not just because it’s old. For Linux with kernel 2.6.19 and newer, osType: Generic with enableParavirtualization: false is a fit, because there’s no four-device limit there and attaching disks on the fly through VirtualMachineBlockDeviceAttachment remains available.

For Legacy, the following isn’t available:

• Changing the number of CPU cores and the amount of memory on a running VM, because these guest operating systems don’t bring them into service. The change is accepted, the VM shows it in .status.restartAwaitingChanges along with the AwaitingRestartToApplyConfiguration condition, and it applies after a restart.
• Changing the contents of .spec.blockDeviceRefs on a running VM, because these disks stay static in both paravirtualization modes, so add a block device before the VM starts.
• The EFI and EFIWithSecureBoot bootloaders, as well as initialization (cloud-init and Sysprep), because these guest operating systems don’t support them.
• Guest OS information in the VM status and file system information.

You can install the QEMU guest agent in such an OS from an archived virtio-win release, and the VM does show AgentReady, but its version is too old for Deckhouse Platform (DP). The VM gets the AgentVersionNotSupported condition, guest OS information isn’t collected, and there’s nothing to update the agent to.

A snapshot with requiredConsistency: true also doesn’t complete successfully, but for a different reason. DP requests a file system freeze, and the agent replies that the command is disabled in its build with the guest-fsfreeze-status has been disabled for this instance message. On Windows, the freeze goes through the VSS provider, which isn’t in this build. The snapshot waits in the InProgress phase for about ten minutes and moves to Failed, so set requiredConsistency: false for such VMs.

With enableParavirtualization: false, one more limitation applies. There can be no more than four block devices in total, because the IDE bus provides two channels with two devices each.

Example configuration for a Windows XP virtual machine:

spec:
  osType: Legacy
  bootloader: BIOS
  enableParavirtualization: false
  # other parameters...

The bootloader parameter defines the bootloader type of the virtual machine:

• BIOS (default): Uses the legacy BIOS.
• EFI: Uses the Unified Extensible Firmware Interface (UEFI/EFI).
  • EFIWithSecureBoot: Uses UEFI/EFI with Secure Boot support.

Example configuration for a Windows virtual machine:

spec:
  osType: Windows
  bootloader: EFI
  # other parameters...

Example configuration for a Linux virtual machine (you can omit the default values):

spec:
  osType: Generic
  bootloader: BIOS
  # other parameters...

The enableParavirtualization parameter controls the use of the virtio bus for attaching the virtual devices of the VM. A change to this parameter takes effect only after the VM restarts.

• true (default): The virtio bus is used for disks, network interfaces, and other devices, which gives better performance. You can change the contents of .spec.blockDeviceRefs on a running VM without a restart, adding and removing devices, if the disk is available on the node where the VM runs.
• false: Standard device emulation is used (SATA for disks, e1000 for network interfaces; IDE and RTL8139 for the Legacy OS type), which may be required for compatibility with older operating systems that lack VirtIO drivers. Changes to .spec.blockDeviceRefs on a running VM (adding and removing disks and images, including ISO) take effect after the VM restarts. To attach and detach disks without a restart, use the VirtualMachineBlockDeviceAttachment (vmbda) resource, without changing the list in the VM specification.

For the Legacy OS type, the default value true isn’t a fit, because these operating systems have no built-in virtio drivers, so for a VM you’re going to install from the original media, set enableParavirtualization: false, otherwise the installer reports that it found no hard drives. A warning is issued when creating and modifying such a VM.

Keep enableParavirtualization: true for a VM with the Legacy OS type only when the virtio drivers are already installed in the guest OS. Then the VM keeps the i440fx chipset and the BIOS bootloader, but gets disks on virtio-blk, the virtio-net adapter, and loses the four-device limit. The CD-ROM stays on the IDE bus, because virtio-blk has no drive.

To switch a system that’s already installed:

1. Install the virtio drivers in the guest OS. For Windows XP, 2000, and Server 2003, take an archived virtio-win release, because the current releases no longer contain drivers for these systems, and install viostor, the virtio-blk driver. The package has no virtio-scsi driver for these operating systems.
2. Power off the VM.
3. Set enableParavirtualization: true.
4. Start the VM.

The order of the steps matters. The disk controller driver has to appear in the guest OS before the switch, not after, because the VM boots from that very controller. If the VM doesn’t boot after the switch, set enableParavirtualization: false back and restart it. The disks return to the IDE bus and the guest OS boots as before.

Example configuration with paravirtualization disabled:

spec:
  enableParavirtualization: false
  # other parameters...

Additional resources

• Module virtualization custom resources