QEMU

QEMU
	The free operating system ReactOS running within QEMU, which runs as a process on Linux
Original author(s)	Fabrice Bellard
Developer(s)	QEMU team:; Peter Maydell, et al.
Stable release	2.6.2 / September 29, 2016; 8 years ago
Repository	gitlab.com/qemu-project/qemu ;
Written in	C
Operating system	Linux, Microsoft Windows, Mac OS X and some other UNIX platforms
Type	Hypervisor
License	GNU GPL version 2
Website	qemu.org

QEMU (short for Quick Emulator) is a free and open-source hosted hypervisor that performs hardware virtualization (not to be confused with hardware-assisted virtualization).

QEMU is a hosted virtual machine monitor: It emulates CPUs through dynamic binary translation and provides a set of device models, enabling it to run a variety of unmodified guest operating systems. It also can be used together with KVM in order to run virtual machines at near-native speed (requiring hardware virtualization extensions on x86 machines). QEMU can also be used purely for CPU emulation for user-level processes, allowing applications compiled for one architecture to be run on another.

Licensing

QEMU was written by Fabrice Bellard and is free software and is mainly licensed under GNU General Public License (GPL). Various parts are released under BSD license, GNU Lesser General Public License (LGPL) or other GPL-compatible licenses.^[2] There is an option to use the proprietary FMOD library when running on Microsoft Windows, which, if used, disqualifies the use of a single open source software license. However, the default is to use DirectSound.

Operating modes

QEMU has multiple operating modes:^[3]

User-mode emulation: In this mode QEMU runs single Linux or Darwin/Mac OS X programs that were compiled for a different instruction set. System calls are thunked for endianness and for 32/64 bit mismatches. Fast cross-compilation and cross-debugging are the main targets for user-mode emulation.
System emulation: In this mode QEMU emulates a full computer system, including peripherals. It can be used to provide virtual hosting of several virtual computers on a single computer. QEMU can boot many guest operating systems, including Linux, Solaris, Microsoft Windows, DOS, and BSD;^[4] it supports emulating several instruction sets, including x86, MIPS, 32-bit ARMv7, ARMv8, PowerPC, SPARC, ETRAX CRIS and MicroBlaze.
KVM Hosting: Here QEMU deals with the setting up and migration of KVM images. It is still involved in the emulation of hardware, but the execution of the guest is done by KVM as requested by QEMU.
Xen Hosting: QEMU is involved only in the emulation of hardware; the execution of the guest is done within Xen and is totally hidden from QEMU.

Features

QEMU can save and restore the state of the virtual machine with all programs running. Guest operating-systems do not need patching in order to run inside QEMU.

QEMU supports the emulation of various architectures, including:

IA-32 (x86) PCs
x86-64 PCs
MIPS R6^[5] and earlier variants
Sun's SPARC sun4m
Sun's SPARC sun4u
ARM development boards (Integrator/CP and Versatile/PB)
SH4 SHIX board
PowerPC (PReP and Power Macintosh)
ETRAX CRIS
MicroBlaze

The virtual machine can interface with many types of physical host hardware. These include: hard disks, CD-ROM drives, network cards, audio interfaces, and USB devices. USB devices can be completely emulated (mass storage from image files, input devices), or the host's USB devices can be used (however, this requires administrator privileges and does not work with all devices).

Virtual disk images can be stored in a special format (qcow or qcow2) that only take up disk space that the guest OS actually uses. This way, an emulated 120 GB disk may occupy only a few hundred megabytes on the host. The QCOW2 format also allows the creation of overlay images that record the difference from another (unmodified) base image file. This provides the possibility for reverting the emulated disk's contents to an earlier state. For example, a base image could hold a fresh install of an operating system that is known to work, and the overlay images are used. Should the guest system become unusable (through virus attack, accidental system destruction, Windows X, ...), the user can delete the overlay and reconstruct an earlier emulated disk-image version.

QEMU can emulate network cards (of different models) which share the host system's connectivity by doing network address translation, effectively allowing the guest to use the same network as the host. The virtual network cards can also connect to network cards of other instances of QEMU or to local TAP interfaces. Network connectivity can also be achieved by bridging a TUN/TAP interface used by QEMU with a non-virtual Ethernet interface on the host OS using the host OS's bridging features.

QEMU integrates several services to allow the host and guest systems to communicate; for example, an integrated SMB server and network-port redirection (to allow incoming connections to the virtual machine). It can also boot Linux kernels without a bootloader.

QEMU does not depend on the presence of graphical output methods on the host system. Instead, it can allow one to access the screen of the guest OS via an integrated VNC server. It can also use an emulated serial line, without any screen, with applicable operating systems.

Simulating multiple CPUs running SMP is possible.

QEMU does not require administrative rights to run, unless additional kernel modules for improving speed are used (like KQEMU), or when some modes of its network connectivity model are utilized.

Tiny Code Generator

The Tiny Code Generator (TCG) aims to remove the shortcoming of relying on a particular version of GCC or any compiler, instead incorporating the compiler (code generator) into other tasks performed by QEMU at run time. The whole translation task thus consists of two parts: blocks of target code (TBs) being rewritten in TCG ops - a kind of machine-independent intermediate notation, and subsequently this notation being compiled for the host's architecture by TCG. Optional optimisation passes are performed between them.

TCG requires dedicated code written to support every architecture it runs on. It also requires that the target instruction translation be rewritten to take advantage of TCG ops, instead of the previously used dyngen ops.

Starting with QEMU Version 0.10.0, TCG ships with the QEMU stable release.^[6]

Accelerator

KQEMU was a Linux kernel module, also written by Fabrice Bellard, which notably speed up emulation of x86 or x86-64 guests on platforms with the same CPU architecture. This worked by running user mode code (and optionally some kernel code) directly on the host computer's CPU, and by using processor and peripheral emulation only for kernel-mode and real-mode code. KQEMU could execute code from many guest OSes even if the host CPU did not support hardware-assisted virtualization. KQEMU was initially a closed-source product available free of charge, but starting from version 1.3.0pre10,^[7] it was relicensed under the GNU General Public License. QEMU versions starting with 0.12.0 (as of August 2009^[update]) support large memory which makes them incompatible with KQEMU.^[8] Newer releases of QEMU have completely removed support for KQEMU.

QVM86 was a GNU GPLv2 licensed drop-in replacement for the then closed-source KQEMU. The developers of QVM86 ceased development in January, 2007.

Kernel-based Virtual Machine (KVM) has mostly taken over as the Linux-based hardware-assisted virtualization solution for use with QEMU in the wake of the lack of support for KQEMU and QVM86.^{[citation needed]}

Intel's Hardware Accelerated Execution Manager (HAXM) is a cost-free (but not open-source) alternative to KVM for x86-based hardware-assisted virtualization on Windows and Mac OS X. As of 2013^[update] Intel mostly solicits its use with QEMU for Android development.^[9]

Supported disk image formats

QEMU supports the following disk image formats:^[10]

OS X Universal Disk Image Format (.dmg) – Read-only
Bochs – Read-only
Linux cloop – Read-only
Parallels disk image (.hdd, .hds) – Read-only
QEMU copy-on-write (.qcow2, .qed, .qcow, .cow)
VirtualBox Virtual Disk Image (.vdi)
Virtual PC Virtual Hard Disk (.vhd)
Virtual VFAT
VMware Virtual Machine Disk (.vmdk)
Raw images (.img) that contain sector-by-sector contents of a disk
CD/DVD images (.iso) that contain sector-by-sector contents of an optical disk (e.g. booting live OSes^[11])

Hardware-assisted emulation

The MIPS-compatible Loongson-3 processor adds 200 new instructions to help QEMU translate x86 instructions; those new instructions lower the overhead of executing x86/CISC-style instructions in the MIPS pipeline. With additional improvements in QEMU by the Chinese Academy of Sciences, Loongson-3 achieves an average of 70% the performance of executing native binaries while running x86 binaries from nine benchmarks.^[12]

Parallel emulation

Virtualization solutions that use QEMU are able to execute multiple virtual CPUs in parallel. QEMU is also able to run multiple threads in user-mode emulation mode.

For full system emulation, QEMU uses a single thread to emulate all the virtual CPUs and hardware. COREMU^[13] is a patch to QEMU to eliminate this limitation. Each core uses a separate instance of QEMU binary translation engine, with a thin library layer to handle the inter-core and device communication and synchronization.

Integration with other virtualization solutions

VirtualBox

VirtualBox, released in January 2007, uses some of QEMU's virtual hardware devices, and has a built-in dynamic recompiler based on QEMU. As with KQEMU, VirtualBox runs nearly all guest code natively on the host via the VMM (Virtual Machine Manager) and uses the recompiler only as a fallback mechanism - for example, when guest code executes in real mode.^[14] In addition, VirtualBox does a lot of code analysis and patching using a built-in disassembler in order to minimize recompilation. VirtualBox is free and open-source (available under GPL), except for certain features.

Xen-HVM

Xen, a virtual machine monitor, can run in HVM (hardware virtual machine) mode, using Intel VT-x or AMD-V hardware x86 virtualization extensions and ARM Cortex-A7 and Cortex-A15 virtualization extension.^[15] This means that instead of paravirtualized devices, a real set of virtual hardware is exposed to the domU to use real device drivers to talk to.

QEMU includes several components: CPU emulators, emulated devices, generic devices, machine descriptions, user interface, and a debugger. The emulated devices and generic devices in QEMU make up its device models for I/O virtualization.^[16] They comprise a PIIX3 IDE (with some rudimentary PIIX4 capabilities), Cirrus Logic or plain VGA emulated video, RTL8139 or E1000 network emulation, and ACPI support.^[17] APIC support is provided by Xen.

Xen-HVM has device emulation based on the QEMU project to provide I/O virtualization to the VMs. Hardware is emulated via a QEMU "device model" daemon running as a backend in dom0. Unlike other QEMU running modes (dynamic translation or KVM), virtual CPUs are completely managed to the hypervisor, which takes care of stopping them while QEMU is emulating memory-mapped I/O accesses.

KVM

KVM (Kernel-based Virtual Machine) is a FreeBSD and Linux kernel module that allows a user space program access to the hardware virtualization features of various processors, with which QEMU is able to offer virtualization for x86, PowerPC, and S/390 guests. When the target architecture is the same as the host architecture, QEMU can make use of KVM particular features, such as acceleration.

Win4Lin Pro Desktop

In early 2005, Win4Lin introduced Win4Lin Pro Desktop, based on a 'tuned' version of QEMU and KQEMU and it hosts NT-versions of Windows. In June 2006,^[18] Win4Lin released Win4Lin Virtual Desktop Server based on the same code base. Win4Lin Virtual Desktop Server serves Microsoft Windows sessions to thin clients from a Linux server.

In September 2006, Win4Lin announced a change of the company name to Virtual Bridges with the release of Win4BSD Pro Desktop, a port of the product to FreeBSD and PC-BSD. Solaris support followed in May 2007 with the release of Win4Solaris Pro Desktop and Win4Solaris Virtual Desktop Server.^[19]

SerialICE

SerialICE is a QEMU-based firmware debugging tool running system firmware inside of QEMU while accessing real hardware through a serial connection to a host system. This can be used as a cheap replacement for hardware ICEs.^[20]

WinUAE

WinUAE Amiga emulator introduced in version 3.0.0 the support for CyberStorm PPC and Blizzard 603e boards using QEMU PPC core.^[21]

Shortcomings

For unaccelerated full system emulation, QEMU cannot utilize multiple host CPUs but runs all the virtual CPUs from a single thread. Virtual CPUs will use multiple threads (and thus can run concurrently) when using KVM for acceleration.

Emulated hardware platforms

x86

Besides the CPU (which is also configurable and can emulate the Intel Sandy Bridge^[22]), the following devices are emulated:

CD-ROM/DVD-drive using an ISO image
Floppy disk
ATA controller or Serial ATA AHCI controller
Graphics card (Cirrus CLGD 5446 PCI VGA-card or Standard-VGA graphics card with Bochs-VESA-BIOS-Extensions - Hardware level, including all non-standard modes, and an experimental patch that can accelerate simple 3D graphics via OpenGL)
Network card (Realtek 8139C+ PCI network adapter)
- Depending on target: i82551, i82557b, i82559er, NE2000 PCI, NE2000 ISA, PCnet, RTL8139, e1000 (PCI Intel Gigabit Ethernet), SMC91c111, Lance and mcf_fec.^[23]^[24]
NVMe disk interface
Parallel port
PC speaker
i440FX/PIIX3 (PCI and ISA) or Q35/ICH9 (PCIe and LPC) chipsets
PS/2-mouse and -keyboard
SCSI controller (LSI MegaRAID SAS 1078, LSI53C895A, NCR53C9x as found in the AMD PCscsi and Tekram DC-390 controllers)
Serial interface
Sound card (Sound Blaster 16, ES1370 PCI, Gravis Ultrasound, and Intel HD Audio^[25])
Watchdog timer (Intel 6300 ESB PCI, or iB700 ISA)
USB 1.x/2.x/3.x controllers (UHCI, EHCI, xHCI)
USB devices: audio, Bluetooth dongle, HID (keyboard/mouse/tablet), MTP, serial interface, CAC smartcard reader, storage (bulk-only transfer and USB Attached SCSI), Wacom tablet
Paravirtualized VirtIO devices: block device, network card, SCSI controller, serial interface, balloon driver, 9pfs filesystem driver
Paravirtualized Xen devices: block device, network card, console, framebuffer and input device

The BIOS implementation used by QEMU starting from version 0.12 is SeaBIOS. The VGA BIOS implementation comes from Plex86/Bochs.

PowerPC

PowerMac

QEMU emulates the following PowerMac peripherals:

UniNorth PCI bridge
PCI-VGA-compatible graphics card which maps the VESA Bochs Extensions
Two PMAC-IDE-Interfaces with hard disk and CD-ROM support.
NE2000 PCI adapter
Non-volatile RAM
VIA-CUDA with ADB keyboard and mouse.

OpenBIOS is used as the firmware.

PREP

QEMU emulates the following PREP peripherals:

PCI bridge
PCI VGA-compatible graphics card with VESA Bochs Extensions
Two IDE interfaces with hard disk and CD-ROM support
Floppy disk drive
NE2000 network adapter
Serial interface
PREP non-volatile RAM
PC-compatible keyboard and mouse

On the PREP target, Open Hack'Ware, an Open-Firmware–compatible BIOS, is used.

IBM System p

QEMU can emulate the paravirtual sPAPR interface with the following peripherals:

PCI bridge, for access to virtio devices, VGA-compatible graphics, USB, etc.
Virtual I/O network adapter, SCSI controller, and serial interface
sPAPR non-volatile RAM

On the sPAPR target, another Open-Firmware–compatible BIOS is used, called SLOF.

ARM

QEMU emulates the ARMv7 instruction set (and down to ARMv5TEJ) with NEON extension.^[26] It emulates full systems like Integrator/CP board, Versatile baseboard, RealView Emulation baseboard, XScale-based PDAs, Palm Tungsten|E PDA, Nokia N800 and Nokia N810 Internet tablets etc. QEMU also powers the Android emulator which is part of the Android SDK (most current Android implementations are ARM based). Starting from version 2.0.0 of their Bada SDK, Samsung has chosen QEMU to help development on emulated 'Wave' devices.

In 1.5.0 and 1.6.0 Samsung Exynos 4210 (dual-core Cortex a9) and Versatile Express ARM Cortex-A9 ARM Cortex-A15 are emulated. In 1.6.0, the 32-bit instructions of the ARMv8 (AARCH64) architecture are emulated, but 64-bit instructions are unsupported.

The Xilinx Cortex A9-based Zynq SoC is modelled, with the following elements:

Zynq-7000 ARM Cortex-A9 CPU
Zynq-7000 ARM Cortex-A9 MPCore
Triple Timer Counter
DDR Memory Controller
DMA Controller (PL330)
Static Memory Controller (NAND/NOR Flash)
SD/SDIO Peripheral Controller (SDHCI)
Zynq Gigabit Ethernet Controller
USB Controller (EHCI - Host support only)
Zynq UART Controller
SPI and QSPI Controllers
I2C Controller

SPARC

QEMU has support for both 32 and 64-bit SPARC architectures.

When the firmware in the JavaStation (sun4m-Architecture) became version 0.8.1 Proll,^[27] a PROM replacement used in version 0.8.2, was replaced with OpenBIOS.

Sparc32

QEMU emulates the following sun4m/sun4c/sun4d peripherals:

IOMMU or IO-UNITs
TCX Frame buffer (graphics card)
Lance (Am7990) Ethernet
Non-volatile RAM M48T02/M48T08
Slave I/O: timers, interrupt controllers, Zilog serial ports, keyboard and power/reset logic
ESP SCSI controller with hard disk and CD-ROM support
Floppy drive (not on SS-600MP)
CS4231 sound device (only on SS-5, not working yet)

Sparc64

Emulating Sun4u (UltraSPARC PC-like machine), Sun4v (T1 PC-like machine), or generic Niagara (T1) machine with the following peripherals:

UltraSparc IIi APB PCI Bridge
PCI VGA compatible card with VESA Bochs Extensions
PS/2 mouse and keyboard
Non-volatile RAM M48T59
PC-compatible serial ports
2 PCI IDE interfaces with hard disk and CD-ROM support
Floppy disk

MicroBlaze

Supported peripherals:

MicroBlaze with/without MMU, including
AXI Timer and Interrupt controller peripherals
AXI External Memory Controller
AXI DMA Controller
Xilinx AXI Ethernet
AXI Ethernet Lite
AXI UART 16650 and UARTLite
AXI SPI Controller

LatticeMico32