Introduction
============

.. _Accelerators:

Virtualisation Accelerators
---------------------------

QEMU's system emulation provides a virtual model of a machine (CPU,
memory and emulated devices) to run a guest OS. It supports a number
of hypervisors (known as accelerators) as well as a JIT known as the
Tiny Code Generator (TCG) capable of emulating many CPUs.

.. list-table:: Supported Accelerators
  :header-rows: 1

  * - Accelerator
    - Host OS
    - Host Architectures
  * - KVM
    - Linux
    - Arm (64 bit only), MIPS, PPC, RISC-V, s390x, x86
  * - Xen
    - Linux (as dom0)
    - Arm, x86
  * - MSHV
    - Linux (as dom0)
    - x86
  * - Hypervisor Framework (hvf)
    - MacOS
    - x86 (64 bit only), Arm (64 bit only)
  * - Windows Hypervisor Platform (whpx)
    - Windows
    - x86
  * - NetBSD Virtual Machine Monitor (nvmm)
    - NetBSD
    - x86
  * - Tiny Code Generator (tcg)
    - Linux, other POSIX, Windows, MacOS
    - Arm, x86, Loongarch64, MIPS, PPC, s390x, Sparc64

Feature Overview
----------------

System emulation provides a wide range of device models to emulate
various hardware components you may want to add to your machine. This
includes a wide number of VirtIO devices which are specifically tuned
for efficient operation under virtualisation. Some of the device
emulation can be offloaded from the main QEMU process using either
vhost-user (for VirtIO) or :ref:`Multi-process QEMU`. If the platform
supports it QEMU also supports directly passing devices through to
guest VMs to eliminate the device emulation overhead. See
:ref:`device-emulation` for more details.

There is a full :ref:`featured block layer<Live Block Operations>`
which allows for construction of complex storage topology which can be
stacked across multiple layers supporting redirection, networking,
snapshots and migration support.

The flexible ``chardev`` system allows for handling IO from character
like devices using stdio, files, unix sockets and TCP networking.

QEMU provides a number of management interfaces including a line based
:ref:`Human Monitor Protocol (HMP)<QEMU monitor>` that allows you to
dynamically add and remove devices as well as introspect the system
state. The :ref:`QEMU Monitor Protocol<QMP Ref>` (QMP) is a well
defined, versioned, machine usable API that presents a rich interface
to other tools to create, control and manage Virtual Machines. This is
the interface used by higher level tools interfaces such as `Virt
Manager <https://virt-manager.org/>`_ using the `libvirt framework
<https://libvirt.org>`_.

For the common accelerators QEMU, supported debugging with its
:ref:`gdbstub<GDB usage>` which allows users to connect GDB and debug
system software images.

Running
-------

QEMU provides a rich and complex API which can be overwhelming to
understand. While some architectures can boot something with just a
disk image, those examples elide a lot of details with defaults that
may not be optimal for modern systems.

For a non-x86 system where we emulate a broad range of machine types,
the command lines are generally more explicit in defining the machine
and boot behaviour. You will often find example command lines in
the :ref:`system-targets-ref` section of the manual.

While the project doesn't want to discourage users from using the
command line to launch VMs, we do want to highlight that there are a
number of projects dedicated to providing a more user friendly
experience. Those built around the ``libvirt`` framework can make use
of feature probing to build modern VM images tailored to run on the
hardware you have.

That said, the general form of a QEMU command line can be expressed
as:

.. parsed-literal::

  $ |qemu_system| [machine opts] \\
                  [cpu opts] \\
                  [accelerator opts] \\
                  [device opts] \\
                  [backend opts] \\
                  [interface opts] \\
                  [boot opts]

Most options will generate some help information. So for example:

.. parsed-literal::

   $ |qemu_system| -M help

will list the machine types supported by that QEMU binary. ``help``
can also be passed as an argument to another option. For example:

.. parsed-literal::

  $ |qemu_system| -device scsi-hd,help

will list the arguments and their default values of additional options
that can control the behaviour of the ``scsi-hd`` device.

.. list-table:: Options Overview
  :header-rows: 1
  :widths: 10, 90

  * - Options
    -
  * - Machine
    - Define the machine type, amount of memory etc
  * - CPU
    - Type and number/topology of vCPUs. Most accelerators offer
      a ``host`` cpu option which simply passes through your host CPU
      configuration without filtering out any features.
  * - Accelerator
    - This will depend on the hypervisor you run. Note that the
      default is TCG, which is purely emulated, so you must specify an
      accelerator type to take advantage of hardware virtualization.
  * - Devices
    - Additional devices that are not defined by default with the
      machine type.
  * - Backends
    - Backends are how QEMU deals with the guest's data, for example
      how a block device is stored, how network devices see the
      network or how a serial device is directed to the outside world.
  * - Interfaces
    - How the system is displayed, how it is managed and controlled or
      debugged.
  * - Boot
    - How the system boots, via firmware or direct kernel boot.

In the following example we first define a ``virt`` machine which is a
general purpose platform for running Aarch64 guests. We enable
virtualisation so we can use KVM inside the emulated guest. As the
``virt`` machine comes with some built in pflash devices we give them
names so we can override the defaults later.

.. code::

 $ qemu-system-aarch64 \
    -machine type=virt,virtualization=on,pflash0=rom,pflash1=efivars \
    -m 4096 \

We then define the 4 vCPUs using the ``max`` option which gives us all
the Arm features QEMU is capable of emulating. We enable a more
emulation friendly implementation of Arm's pointer authentication
algorithm. We explicitly specify TCG acceleration even though QEMU
would default to it anyway.

.. code::

 -cpu max \
 -smp 4 \
 -accel tcg \

As the ``virt`` platform doesn't have any default network or storage
devices we need to define them. We give them ids so we can link them
with the backend later on.

.. code::

 -device virtio-net-pci,netdev=unet \
 -device virtio-scsi-pci \
 -device scsi-hd,drive=hd \

We connect the user-mode networking to our network device. As
user-mode networking isn't directly accessible from the outside world
we forward localhost port 2222 to the ssh port on the guest.

.. code::

 -netdev user,id=unet,hostfwd=tcp::2222-:22 \

We connect the guest visible block device to an LVM partition we have
set aside for our guest.

.. code::

 -blockdev driver=raw,node-name=hd,file.driver=host_device,file.filename=/dev/lvm-disk/debian-bullseye-arm64 \

We then tell QEMU to multiplex the :ref:`QEMU monitor` with the serial
port output (we can switch between the two using :ref:`keys in the
character backend multiplexer`). As there is no default graphical
device we disable the display as we can work entirely in the terminal.

.. code::

 -serial mon:stdio \
 -display none \

Finally we override the default firmware to ensure we have some
storage for EFI to persist its configuration. That firmware is
responsible for finding the disk, booting grub and eventually running
our system.

.. code::

 -blockdev node-name=rom,driver=file,filename=(pwd)/pc-bios/edk2-aarch64-code.fd,read-only=true \
 -blockdev node-name=efivars,driver=file,filename=$HOME/images/qemu-arm64-efivars