QEMU

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QEMU system emulation running Windows 2000 on Red Hat Linux.

QEMU (Quick Emulator) is a free, open-source hypervisor and emulator that runs on several operating systems, including many major Linux distros, macOS, Microsoft Windows, BSD, as well as a few others.

It is capable of running pretty much all major x86 operating systems, including Windows, Linux, MS-DOS and BSD (and much more), and in addition is also capable of emulating other architectures, including ARM, MIPS, SPARC, PowerPC and MicroBlaze. There are also many forks of QEMU that add emulation of other systems and processors, like the Xbox, Zilog Z80 and Macintosh 128K.

QEMU is the only emulator supported in CollabVM 1.2. CollabVM 2.0 has a wider selection of virtualizers and emulators.

Operating modes

User emulation

User emulation mode emulates a target CPU (always using the Tiny Code Generator), and additionally a given system call ABI (usually Linux), allowing programs from another architecture or operating system to run as if they are native applications (in most cases). This is useful for poking around embedded Linux firmware, for instance, but beyond that generally is more of a curiosity than something useful.

System emulation

System emulation mode (known also as softmmu) emulates an entire computer system, including peripherals. As mentioned above, it is capable of booting practically all major operating systems. By default, system emulation uses the Tiny Code Generator (TCG), which is generally slower (although faster than Bochs, at a cost of accuracy).

TCG should only be used if:

  • The host server does not have KVM/virtualization enabled.
  • The host server is running an OpenVZ variant of Linux.
  • You are NOT in the kvm user group, or have not started the kvm module on the OS.
  • The host operating system does not run well with KVM (e.g. Windows 98 does not run with KVM on most systems)

System emulation under a hypervisor

Alternatively, QEMU can use a hypervisor API provided by the host operating system to run the virtual CPUs (vCPUs), which allows it to run operating systems at near native speed.

When a given hypervisor is enabled, instead of using TCG to emulate each vCPU present in the configured machine, QEMU will instead request the hypervisor run the vCPU, which will use native virtualization features that are a part of the CPU.

Most of QEMU's hardware/system emulation still runs in usermode with a hypervisor enabled. On KVM, chipset emulation functionality is by default moved into the kernel, helping increase performance further.

QEMU supports the following hypervisor APIs:

Hypervisor APIs
API Platforms Details
KVM (-accel kvm) Linux Linux's native hypervisor API, which works on x86_64, ppc, arm, aarch64, s390x and others. You can check for KVM support by running the command kvm-ok, or alternatively ls /dev/kvm.
HAXM (-accel haxm) Windows, macOS, FreeBSD, Linux A deprecated hypervisor layer written by Intel for x86 platforms. Not recommended for use.
WHPX (-accel whpx) Windows Windows Hypervisor Platform. Essentially the hypervisor core of Hyper-V turned into an API. This does not run very well, and certain OSes do not run properly or very slowly under it.
GVM/AEHD (-accel kvm) Windows Google-backed port of KVM to Windows, mainly used for the Android Virtual Device emulator, which is based on QEMU. Needs a fork of QEMU patched to support it, and is rather unstable.
Hypervisor.framework (-accel hvf) macOS macOS specific hypervisor API.

Note that you can also use -M ...,accel=<hypervisor> to enable a given hypervisor as well.

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