QEMU CPU Emulator User Documentation

Table of Contents

QEMU CPU Emulator User Documentation

1. Introduction

1.1 Features

QEMU is a FAST! processor emulator using dynamic translation to achieve good emulation speed.

QEMU has two operating modes:

As QEMU requires no host kernel driver to run, it is very safe and easy to use.

For system emulation, the following hardware targets are supported:

For user emulation, x86, PowerPC, ARM, and SPARC CPUs are supported.

2. Installation

If you want to compile QEMU yourself, see section 6. Compilation from the sources.

2.1 Linux

Download the binary distribution (`qemu-XXX-i386.tar.gz') and untar it as root in `/': 

su
cd /
tar zxvf /tmp/qemu-XXX-i386.tar.gz

2.2 Windows

Download the experimental binary installer at http://www.freeoszoo.org/download.php.

2.3 Mac OS X

Download the experimental binary installer at http://www.freeoszoo.org/download.php.

3. QEMU PC System emulator invocation

3.1 Introduction

The QEMU System emulator simulates a complete PC.

In order to meet specific user needs, two versions of QEMU are available:

  1. qemu-fast uses the host Memory Management Unit (MMU) to simulate the x86 MMU. It is fast but has limitations because the whole 4 GB address space cannot be used and some memory mapped peripherials cannot be emulated accurately yet. Therefore, a specific guest Linux kernel can be used (See section 3.8 Linux Kernel Compilation) as guest OS.
  2. qemu uses a software MMU. It is about two times slower but gives a more accurate emulation.

QEMU emulates the following PC peripherials:

QEMU uses the PC BIOS from the Bochs project and the Plex86/Bochs LGPL VGA BIOS.

3.2 Quick Start

Download and uncompress the linux image (`linux.img') and type: 

qemu linux.img

Linux should boot and give you a prompt.

3.3 Invocation

usage: qemu [options] [disk_image]

disk_image is a raw hard disk image for IDE hard disk 0.

General options:

`-fda file'
`-fdb file'
Use file as floppy disk 0/1 image (See section 3.5 Disk Images). You can use the host floppy by using `/dev/fd0' as filename.
`-hda file'
`-hdb file'
`-hdc file'
`-hdd file'
Use file as hard disk 0, 1, 2 or 3 image (See section 3.5 Disk Images).
`-cdrom file'
Use file as CD-ROM image (you cannot use `-hdc' and and `-cdrom' at the same time). You can use the host CD-ROM by using `/dev/cdrom' as filename.
`-boot [a|c|d]'
Boot on floppy (a), hard disk (c) or CD-ROM (d). Hard disk boot is the default.
`-snapshot'
Write to temporary files instead of disk image files. In this case, the raw disk image you use is not written back. You can however force the write back by pressing C-a s (See section 3.5 Disk Images).
`-m megs'
Set virtual RAM size to megs megabytes. Default is 128 MB.
`-initrd file'
Use file as initial ram disk.
`-nographic'
Normally, QEMU uses SDL to display the VGA output. With this option, you can totally disable graphical output so that QEMU is a simple command line application. The emulated serial port is redirected on the console. Therefore, you can still use QEMU to debug a Linux kernel with a serial console.
`-enable-audio'
The SB16 emulation is disabled by default as it may give problems with Windows. You can enable it manually with this option.
`-localtime'
Set the real time clock to local time (the default is to UTC time). This option is needed to have correct date in MS-DOS or Windows.

Network options:

`-n script'
Set TUN/TAP network init script [default=/etc/qemu-ifup]. This script is launched to configure the host network interface (usually tun0) corresponding to the virtual NE2000 card.
`-macaddr addr'
Set the mac address of the first interface (the format is aa:bb:cc:dd:ee:ff in hexa). The mac address is incremented for each new network interface.
`-tun-fd fd'
Assumes fd talks to a tap/tun host network interface and use it. Read http://bellard.org/qemu/tetrinet.html to have an example of its use.
`-user-net'
Use the user mode network stack. This is the default if no tun/tap network init script is found.
`-dummy-net'
Use the dummy network stack: no packet will be received by the network cards.

Linux boot specific. When using this options, you can use a given Linux kernel without installing it in the disk image. It can be useful for easier testing of various kernels.

`-kernel bzImage'
Use bzImage as kernel image.
`-append cmdline'
Use cmdline as kernel command line
`-initrd file'
Use file as initial ram disk.

Debug/Expert options:

`-s'
Wait gdb connection to port 1234 (See section 3.9 GDB usage).
`-p port'
Change gdb connection port.
`-S'
Do not start CPU at startup (you must type 'c' in the monitor).
`-d'
Output log in /tmp/qemu.log
`-isa'
Simulate an ISA-only system (default is PCI system).
`-std-vga'
Simulate a standard VGA card with Bochs VBE extensions (default is Cirrus Logic GD5446 PCI VGA)

During the graphical emulation, you can use the following keys:

Ctrl-Shift
Toggle mouse and keyboard grab.
Ctrl-Shift-f
Toggle full screen

During emulation, if you are using the serial console, use C-a h to get terminal commands:

Ctrl-a h
Print this help
Ctrl-a x
Exit emulatior
Ctrl-a s
Save disk data back to file (if -snapshot)
Ctrl-a b
Send break (magic sysrq in Linux)
Ctrl-a c
Switch between console and monitor
Ctrl-a Ctrl-a
Send Ctrl-a

3.4 QEMU Monitor

The QEMU monitor is used to give complex commands to the QEMU emulator. You can use it to:

3.4.1 Commands

The following commands are available:

`help or ? [cmd]'
Show the help for all commands or just for command cmd.
`commit'
Commit changes to the disk images (if -snapshot is used)
`info subcommand'
show various information about the system state
`info network'
show the network state
`info block'
show the block devices
`info registers'
show the cpu registers
`info history'
show the command line history
`q or quit'
Quit the emulator.
`eject [-f] device'
Eject a removable media (use -f to force it).
`change device filename'
Change a removable media.
`screendump filename'
Save screen into PPM image filename.
`log item1[,...]'
Activate logging of the specified items to `/tmp/qemu.log'.
`savevm filename'
Save the whole virtual machine state to filename.
`loadvm filename'
Restore the whole virtual machine state from filename.
`stop'
Stop emulation.
`c or cont'
Resume emulation.
`gdbserver [port]'
Start gdbserver session (default port=1234)
`x/fmt addr'
Virtual memory dump starting at addr.
`xp /fmt addr'
Physical memory dump starting at addr. fmt is a format which tells the command how to format the data. Its syntax is: `/{count}{format}{size}'
count
is the number of items to be dumped.
format
can be x (hexa), d (signed decimal), u (unsigned decimal), o (octal), c (char) or i (asm instruction).
size
can be b (8 bits), h (16 bits), w (32 bits) or g (64 bits). On x86, h or w can be specified with the i format to respectively select 16 or 32 bit code instruction size.
Examples:
`p or print/fmt expr'
Print expression value. Only the format part of fmt is used.
`sendkey keys'
Send keys to the emulator. Use - to press several keys simultaneously. Example: 
sendkey ctrl-alt-f1
This command is useful to send keys that your graphical user interface intercepts at low level, such as ctrl-alt-f1 in X Window.
`system_reset'
Reset the system.

3.4.2 Integer expressions

The monitor understands integers expressions for every integer argument. You can use register names to get the value of specifics CPU registers by prefixing them with $.

3.5 Disk Images

3.5.1 Raw disk images

The disk images can simply be raw images of the hard disk. You can create them with the command: 

dd of=myimage bs=1024 seek=mysize count=0

where myimage is the image filename and mysize is its size in kilobytes.

3.5.2 Snapshot mode

If you use the option `-snapshot', all disk images are considered as read only. When sectors in written, they are written in a temporary file created in `/tmp'. You can however force the write back to the raw disk images by pressing C-a s.

NOTE: The snapshot mode only works with raw disk images.

3.5.3 Copy On Write disk images

QEMU also supports user mode Linux (http://user-mode-linux.sourceforge.net/) Copy On Write (COW) disk images. The COW disk images are much smaller than normal images as they store only modified sectors. They also permit the use of the same disk image template for many users.

To create a COW disk images, use the command: 

qemu-mkcow -f myrawimage.bin mycowimage.cow

`myrawimage.bin' is a raw image you want to use as original disk image. It will never be written to.

`mycowimage.cow' is the COW disk image which is created by qemu-mkcow. You can use it directly with the `-hdx' options. You must not modify the original raw disk image if you use COW images, as COW images only store the modified sectors from the raw disk image. QEMU stores the original raw disk image name and its modified time in the COW disk image so that chances of mistakes are reduced.

If the raw disk image is not read-only, by pressing C-a s you can flush the COW disk image back into the raw disk image, as in snapshot mode.

COW disk images can also be created without a corresponding raw disk image. It is useful to have a big initial virtual disk image without using much disk space. Use: 

qemu-mkcow mycowimage.cow 1024

to create a 1 gigabyte empty COW disk image.

NOTES:

  1. COW disk images must be created on file systems supporting holes such as ext2 or ext3.
  2. Since holes are used, the displayed size of the COW disk image is not the real one. To know it, use the ls -ls command.

3.5.4 Convert VMware disk images to raw disk images

You can use the tool `vmdk2raw' to convert VMware disk images to raw disk images directly usable by QEMU. The syntax is: 

vmdk2raw vmware_image output_image

3.6 Network emulation

QEMU simulates up to 6 networks cards (NE2000 boards). Each card can be connected to a specific host network interface.

3.6.1 Using tun/tap network interface

This is the standard way to emulate network. QEMU adds a virtual network device on your host (called tun0), and you can then configure it as if it was a real ethernet card.

As an example, you can download the `linux-test-xxx.tar.gz' archive and copy the script `qemu-ifup' in `/etc' and configure properly sudo so that the command ifconfig contained in `qemu-ifup' can be executed as root. You must verify that your host kernel supports the TUN/TAP network interfaces: the device `/dev/net/tun' must be present.

See section 3.7 Direct Linux Boot to have an example of network use with a Linux distribution.

3.6.2 Using the user mode network stack

By using the option `-user-net' or if you have no tun/tap init script, QEMU uses a completely user mode network stack (you don't need root priviledge to use the virtual network). The virtual network configuration is the following: 

QEMU Virtual Machine    <------>  Firewall/DHCP server <-----> Internet
     (10.0.2.x)            |          (10.0.2.2)
                           |
                           ---->  DNS 
                              (10.0.2.3)

The QEMU VM behaves as if it was behind a firewall which blocks all incoming connections. You can use a DHCP client to automatically configure the network in the QEMU VM.

In order to check that the user mode network is working, you can ping the address 10.0.2.2 and verify that you got an address in the range 10.0.2.x from the QEMU virtual DHCP server.

Note that ping is not supported reliably to the internet as it would require root priviledges. It means you can only ping the local router (10.0.2.2).

The user mode network is currently only supported on a Unix host.

3.7 Direct Linux Boot

This section explains how to launch a Linux kernel inside QEMU without having to make a full bootable image. It is very useful for fast Linux kernel testing. The QEMU network configuration is also explained.

  1. Download the archive `linux-test-xxx.tar.gz' containing a Linux kernel and a disk image.
  2. Optional: If you want network support (for example to launch X11 examples), you must copy the script `qemu-ifup' in `/etc' and configure properly sudo so that the command ifconfig contained in `qemu-ifup' can be executed as root. You must verify that your host kernel supports the TUN/TAP network interfaces: the device `/dev/net/tun' must be present. When network is enabled, there is a virtual network connection between the host kernel and the emulated kernel. The emulated kernel is seen from the host kernel at IP address 172.20.0.2 and the host kernel is seen from the emulated kernel at IP address 172.20.0.1.
  3. Launch qemu.sh. You should have the following output: 
    > ./qemu.sh 
Connected to host network interface: tun0
Linux version 2.4.21 (bellard@voyager.localdomain) (gcc version 3.2.2 20030222 (Red Hat Linux 3.2.2-5)) #5 Tue Nov 11 18:18:53 CET 2003
BIOS-provided physical RAM map:
 BIOS-e801: 0000000000000000 - 000000000009f000 (usable)
 BIOS-e801: 0000000000100000 - 0000000002000000 (usable)
32MB LOWMEM available.
On node 0 totalpages: 8192
zone(0): 4096 pages.
zone(1): 4096 pages.
zone(2): 0 pages.
Kernel command line: root=/dev/hda sb=0x220,5,1,5 ide2=noprobe ide3=noprobe ide4=noprobe ide5=noprobe console=ttyS0
ide_setup: ide2=noprobe
ide_setup: ide3=noprobe
ide_setup: ide4=noprobe
ide_setup: ide5=noprobe
Initializing CPU#0
Detected 2399.621 MHz processor.
Console: colour EGA 80x25
Calibrating delay loop... 4744.80 BogoMIPS
Memory: 28872k/32768k available (1210k kernel code, 3508k reserved, 266k data, 64k init, 0k highmem)
Dentry cache hash table entries: 4096 (order: 3, 32768 bytes)
Inode cache hash table entries: 2048 (order: 2, 16384 bytes)
Mount cache hash table entries: 512 (order: 0, 4096 bytes)
Buffer-cache hash table entries: 1024 (order: 0, 4096 bytes)
Page-cache hash table entries: 8192 (order: 3, 32768 bytes)
CPU: Intel Pentium Pro stepping 03
Checking 'hlt' instruction... OK.
POSIX conformance testing by UNIFIX
Linux NET4.0 for Linux 2.4
Based upon Swansea University Computer Society NET3.039
Initializing RT netlink socket
apm: BIOS not found.
Starting kswapd
Journalled Block Device driver loaded
Detected PS/2 Mouse Port.
pty: 256 Unix98 ptys configured
Serial driver version 5.05c (2001-07-08) with no serial options enabled
ttyS00 at 0x03f8 (irq = 4) is a 16450
ne.c:v1.10 9/23/94 Donald Becker (becker@scyld.com)
Last modified Nov 1, 2000 by Paul Gortmaker
NE*000 ethercard probe at 0x300: 52 54 00 12 34 56
eth0: NE2000 found at 0x300, using IRQ 9.
RAMDISK driver initialized: 16 RAM disks of 4096K size 1024 blocksize
Uniform Multi-Platform E-IDE driver Revision: 7.00beta4-2.4
ide: Assuming 50MHz system bus speed for PIO modes; override with idebus=xx
hda: QEMU HARDDISK, ATA DISK drive
ide0 at 0x1f0-0x1f7,0x3f6 on irq 14
hda: attached ide-disk driver.
hda: 20480 sectors (10 MB) w/256KiB Cache, CHS=20/16/63
Partition check:
 hda:
Soundblaster audio driver Copyright (C) by Hannu Savolainen 1993-1996
NET4: Linux TCP/IP 1.0 for NET4.0
IP Protocols: ICMP, UDP, TCP, IGMP
IP: routing cache hash table of 512 buckets, 4Kbytes
TCP: Hash tables configured (established 2048 bind 4096)
NET4: Unix domain sockets 1.0/SMP for Linux NET4.0.
EXT2-fs warning: mounting unchecked fs, running e2fsck is recommended
VFS: Mounted root (ext2 filesystem).
Freeing unused kernel memory: 64k freed
 
Linux version 2.4.21 (bellard@voyager.localdomain) (gcc version 3.2.2 20030222 (Red Hat Linux 3.2.2-5)) #5 Tue Nov 11 18:18:53 CET 2003
 
QEMU Linux test distribution (based on Redhat 9)
 
Type 'exit' to halt the system
 
sh-2.05b#
  4. Then you can play with the kernel inside the virtual serial console. You can launch ls for example. Type Ctrl-a h to have an help about the keys you can type inside the virtual serial console. In particular, use Ctrl-a x to exit QEMU and use Ctrl-a b as the Magic SysRq key.
  5. If the network is enabled, launch the script `/etc/linuxrc' in the emulator (don't forget the leading dot): 
    . /etc/linuxrc
    Then enable X11 connections on your PC from the emulated Linux: 
    xhost +172.20.0.2
    You can now launch `xterm' or `xlogo' and verify that you have a real Virtual Linux system !

NOTES:

  1. A 2.5.74 kernel is also included in the archive. Just replace the bzImage in qemu.sh to try it.
  2. qemu-fast creates a temporary file in $QEMU_TMPDIR (`/tmp' is the default) containing all the simulated PC memory. If possible, try to use a temporary directory using the tmpfs filesystem to avoid too many unnecessary disk accesses.
  3. In order to exit cleanly from qemu, you can do a shutdown inside qemu. qemu will automatically exit when the Linux shutdown is done.
  4. You can boot slightly faster by disabling the probe of non present IDE interfaces. To do so, add the following options on the kernel command line: 
    ide1=noprobe ide2=noprobe ide3=noprobe ide4=noprobe ide5=noprobe
  5. The example disk image is a modified version of the one made by Kevin Lawton for the plex86 Project (www.plex86.org).

3.8 Linux Kernel Compilation

You can use any linux kernel with QEMU. However, if you want to use qemu-fast to get maximum performances, you must use a modified guest kernel. If you are using a 2.6 guest kernel, you can use directly the patch `linux-2.6-qemu-fast.patch' made by Rusty Russel available in the QEMU source archive. Otherwise, you can make the following changes by hand to the Linux kernel:

  1. The kernel must be mapped at 0x90000000 (the default is 0xc0000000). You must modify only two lines in the kernel source: In `include/asm/page.h', replace 
    #define __PAGE_OFFSET           (0xc0000000)
    by 
    #define __PAGE_OFFSET           (0x90000000)
    And in `arch/i386/vmlinux.lds', replace 
      . = 0xc0000000 + 0x100000;
    by 
      . = 0x90000000 + 0x100000;
  2. If you want to enable SMP (Symmetric Multi-Processing) support, you must make the following change in `include/asm/fixmap.h'. Replace 
    #define FIXADDR_TOP	(0xffffX000UL)
    by 
    #define FIXADDR_TOP	(0xa7ffX000UL)
    (X is 'e' or 'f' depending on the kernel version). Although you can use an SMP kernel with QEMU, it only supports one CPU.
  3. If you are not using a 2.6 kernel as host kernel but if you use a target 2.6 kernel, you must also ensure that the 'HZ' define is set to 100 (1000 is the default) as QEMU cannot currently emulate timers at frequencies greater than 100 Hz on host Linux systems < 2.6. In `include/asm/param.h', replace: 
    # define HZ		1000		/* Internal kernel timer frequency */
    by 
    # define HZ		100		/* Internal kernel timer frequency */

The file config-2.x.x gives the configuration of the example kernels.

Just type 

make bzImage

As you would do to make a real kernel. Then you can use with QEMU exactly the same kernel as you would boot on your PC (in `arch/i386/boot/bzImage').

3.9 GDB usage

QEMU has a primitive support to work with gdb, so that you can do 'Ctrl-C' while the virtual machine is running and inspect its state.

In order to use gdb, launch qemu with the '-s' option. It will wait for a gdb connection: 

> qemu -s -kernel arch/i386/boot/bzImage -hda root-2.4.20.img -append "root=/dev/hda"
Connected to host network interface: tun0
Waiting gdb connection on port 1234

Then launch gdb on the 'vmlinux' executable: 

> gdb vmlinux

In gdb, connect to QEMU: 

(gdb) target remote localhost:1234

Then you can use gdb normally. For example, type 'c' to launch the kernel: 

(gdb) c

Here are some useful tips in order to use gdb on system code:

  1. Use info reg to display all the CPU registers.
  2. Use x/10i $eip to display the code at the PC position.
  3. Use set architecture i8086 to dump 16 bit code. Then use x/10i $cs*16+*eip to dump the code at the PC position.

3.10 Target OS specific information

3.10.1 Linux

To have access to SVGA graphic modes under X11, use the vesa or the cirrus X11 driver. For optimal performances, use 16 bit color depth in the guest and the host OS.

When using a 2.6 guest Linux kernel, you should add the option clock=pit on the kernel command line because the 2.6 Linux kernels make very strict real time clock checks by default that QEMU cannot simulate exactly.

3.10.2 Windows

If you have a slow host, using Windows 95 is better as it gives the best speed. Windows 2000 is also a good choice.

3.10.2.1 SVGA graphic modes support

QEMU emulates a Cirrus Logic GD5446 Video card. All Windows versions starting from Windows 95 should recognize and use this graphic card. For optimal performances, use 16 bit color depth in the guest and the host OS.

3.10.2.2 CPU usage reduction

Windows 9x does not correctly use the CPU HLT instruction. The result is that it takes host CPU cycles even when idle. You can install the utility from http://www.user.cityline.ru/~maxamn/amnhltm.zip to solve this problem. Note that no such tool is needed for NT, 2000 or XP.

3.10.2.3 Windows 2000 disk full problems

Currently (release 0.6.0) QEMU has a bug which gives a disk full error during installation of some releases of Windows 2000. The workaround is to stop QEMU as soon as you notice that your disk image size is growing too fast (monitor it with ls -ls). Then relaunch QEMU to continue the installation. If you still experience the problem, relaunch QEMU again.

Future QEMU releases are likely to correct this bug.

3.10.2.4 Windows XP security problems

Some releases of Windows XP install correctly but give a security error when booting: 

A problem is preventing Windows from accurately checking the
license for this computer. Error code: 0x800703e6.

The only known workaround is to boot in Safe mode without networking support.

Future QEMU releases are likely to correct this bug.

4. QEMU PowerPC System emulator invocation

Use the executable `qemu-system-ppc' to simulate a complete PREP or PowerMac PowerPC system.

QEMU emulates the following PowerMac peripherials:

QEMU emulates the following PREP peripherials:

QEMU uses the Open Hack'Ware Open Firmware Compatible BIOS available at http://site.voila.fr/jmayer/OpenHackWare/index.htm.

You can read the qemu PC system emulation chapter to have more informations about QEMU usage.

The following options are specific to the PowerPC emulation:

`-prep'
Simulate a PREP system (default is PowerMAC)
`-g WxH[xDEPTH]'
Set the initial VGA graphic mode. The default is 800x600x15.

More information is available at http://jocelyn.mayer.free.fr/qemu-ppc/.

5. QEMU User space emulator invocation

5.1 Quick Start

In order to launch a Linux process, QEMU needs the process executable itself and all the target (x86) dynamic libraries used by it.

5.2 Wine launch

5.3 Command line options

usage: qemu-i386 [-h] [-d] [-L path] [-s size] program [arguments...]
`-h'
Print the help
`-L path'
Set the x86 elf interpreter prefix (default=/usr/local/qemu-i386)
`-s size'
Set the x86 stack size in bytes (default=524288)

Debug options:

`-d'
Activate log (logfile=/tmp/qemu.log)
`-p pagesize'
Act as if the host page size was 'pagesize' bytes

6. Compilation from the sources

6.1 Linux/BSD

Read the `README' which gives the related information.

6.2 Windows

6.3 Cross compilation for Windows with Linux

Note: Currently, Wine does not seem able to launch QEMU for Win32.

6.4 Mac OS X

The Mac OS X patches are not fully merged in QEMU, so you should look at the QEMU mailing list archive to have all the necessary information.

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