9.5 KiB
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. The serial port can be very useful to gather early logs from the operating system booted inside the VM.
For x86_64, The default serial port is from an emulated 16550A device. It can
be used as the default console for Linux when booting with the option
console=ttyS0. For AArch64, the default serial port is from an emulated
PL011 UART device. The related command line for AArch64 is console=ttyAMA0.
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.
For AArch64 machines, an ARM PrimeCell Real Time Clock(PL031) is implemented. This device is built-in by default for the AArch64 platform, and 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.
ARM PrimeCell General Purpose Input/Output (PL061)
Simplified ARM PrimeCell GPIO (PL061) implementation. Only supports key 3 to trigger a graceful shutdown of the AArch64 guest.
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, only virtio-pci transport layer is
supported. Cloud Hypervisor supports multiple PCI segments, and users can
append ,pci_segment=<PCI_segment_number> to the device flag in the Cloud
Hypervisor command line to assign devices to a specific PCI segment.
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 when vhost_user=true and
socket are provided to the --disk parameter.
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 when vhost_user=true and
socket are provided to the --net parameter.
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.