cloud-hypervisor/docs/device_model.md

Device Model

This document describes the device model supported by cloud-hypervisor.

Summary

Device	Build configurable	Enabled by default	Runtime configurable
Serial port	❎	❎	✔️
RTC/CMOS	✔️	✔️	❎
I/O APIC	❎	❎	✔️
i8042 shutdown/reboot	❎	❎	❎
ACPI shutdown/reboot	❎	✔️	❎
virtio-blk	❎	❎	✔️
virtio-console	❎	❎	✔️
virtio-iommu	❎	❎	✔️
virtio-net	❎	❎	✔️
virtio-pmem	❎	❎	✔️
virtio-rng	❎	❎	✔️
virtio-vsock	❎	❎	✔️
vhost-user-blk	❎	❎	✔️
vhost-user-fs	❎	❎	✔️
vhost-user-net	❎	❎	✔️
VFIO	✔️	❎	✔️

Legacy devices

Serial port

Simple emulation of a serial port by reading and writing to specific port I/O addresses. Used as the default console for Linux when booting with the option console=ttyS0, the serial port can be very useful to gather early logs from the operating system booted inside the VM.

This device is always built-in, and it is disabled by default. It can be enabled with the --serial option, as long as its parameter is not off.

RTC/CMOS

For environments such as Windows or EFI which cannot rely on KVM clock, the emulation of this legacy device makes the platform usable.

This device is built-in by default, but it can be compiled out with Rust features. When compiled in, it is always enabled, and cannot be disabled from the command line.

I/O APIC

cloud-hypervisor supports a so-called split IRQ chip implementation by implementing support for the IOAPIC. By moving part of the IRQ chip implementation from kernel space to user space, the IRQ chip emulation does not always run in a fully privileged mode.

The device is always built-in, and it is enabled depending on the presence of the serial port. If the serial port is disabled, and because no other device would require pin based interrupts (INTx), the I/O APIC is disabled.

i8042

Simplified PS/2 port since it supports only one key to trigger a reboot or shutdown, depending on the ACPI support.

This device is always built-in, but it is disabled by default. Because ACPI is enabled by default, the handling of reboot/shutdown goes through the dedicated ACPI device. In case ACPI is disabled, this device is enabled to bring to the VM some reboot/shutdown support.

ACPI device

This is a dedicated device for handling ACPI shutdown and reboot when ACPI is enabled.

This device is always built-in, and it is enabled by default since the ACPI feature is enabled by default.

Virtio devices

For all virtio devices listed below, both virtio-mmio and virtio-pci transport layers are supported, virtio-pci being the default.

Both virtio-mmio and virtio-pci can be compiled out. virtio-pci is built-in by default, and enabled by default. If both transport layers were built at the same time, virtio-pci would be the default transport layer.

virtio-block

The virtio-blk device exposes a block device to the guest. This device is usually used to boot the operating system running in the VM.

This device is always built-in, and it is enabled based on the presence of the flag --disk.

virtio-console

cloud-hypervisor exposes a virtio-console device to the guest. Although using this device as a guest console can potentially cut some early boot messages, it can reduce the guest boot time and provides a complete console implementation.

This device is always built-in, and it is enabled by default to provide a guest console. It can be disabled, switching back to the legacy serial port by selecting --serial tty --console off from the command line.

virtio-iommu

As we want to improve our nested guests support, we added support for exposing a paravirtualized IOMMU device through virtio. This allows for a safer nested virtio and directly assigned devices support.

This device is always built-in, and it is enabled based on the presence of the parameter iommu=on in any of the virtio or VFIO devices. If at least one of these devices needs to be connected to the paravirtualized IOMMU, the virtio-iommu device will be created.

virtio-net

The virtio-net device provides network connectivity for the guest, as it creates a network interface connected to a TAP interface automatically created by the cloud-hypervisor on the host.

This device is always built-in, and it is enabled based on the presence of the flag --net.

virtio-pmem

The virtio-pmem implementation emulates a virtual persistent memory device that cloud-hypervisor can e.g. boot from. Booting from a virtio-pmem device allows to bypass the guest page cache and improve the guest memory footprint.

This device is always built-in, and it is enabled based on the presence of the flag --pmem.

virtio-rng

A VM does not generate entropy like a real machine would, which is an issue when workloads running in the guest need random numbers to be generated. The virtio-rng device provides entropy to the guest by relying on the generator that can be found on the host. By default, the chosen source of entropy is /dev/urandom.

This device is always built-in, and it is always enabled. The --rng flag can be used to change the source of entropy.

virtio-vsock

In order to more efficiently and securely communicate between host and guest, we added a hybrid implementation of the VSOCK socket address family over virtio. Credits go to the Firecracker project as our implementation is a copy of theirs.

This device is always built-in, and it is enabled based on the presence of the flag --vsock.

Vhost-user devices

Vhost-user devices are virtio backends running outside of the VMM, as its own separate process. They are usually used to bring more flexibility and increased isolation.

vhost-user-blk

As part of the general effort to offload paravirtualized I/O to external processes, we added support for vhost-user-blk backends. This enables cloud-hypervisor users to plug a vhost-user based block device (e.g. SPDK) into the VMM as their virtio block backend.

This device is always built-in, and it is enabled based on the presence of the flag --vhost-user-blk.

vhost-user-fs

cloud-hypervisor supports the virtio-fs shared file system, allowing for an efficient and reliable way of sharing a filesystem between the host and the cloud-hypervisor guest.

See our filesystem sharing documentation for more details on how to use virtio-fs with cloud-hypervisor.

This device is always built-in, and it is enabled based on the presence of the flag --fs.

vhost-user-net

As part of the general effort to offload paravirtualized I/O to external processes, we added support for vhost-user-net backends. This enables cloud-hypervisor users to plug a vhost-user based networking device (e.g. DPDK) into the VMM as their virtio network backend.

This device is always built-in, and it is enabled based on the presence of the flag --vhost-user-net.

VFIO

VFIO (Virtual Function I/O) is a kernel framework that exposes direct device access to userspace. cloud-hypervisor uses VFIO to directly assign host physical devices into its guest.

See our VFIO documentation for more details on how to directly assign host devices to cloud-hypervisor guests.

Because VFIO implies vfio-pci in the cloud-hypervisor context, the VFIO support is built-in when the pci feature is selected. And because the pci feature is built-in by default, VFIO support is also built-in by default. When VFIO support is built-in, a physical device can be passed through, using the flag --device in order to enable the VFIO code.