1. Introduction

1.1 Features

QEMU is a FAST! processor emulator. By using dynamic translation it achieves a reasonnable speed while being easy to port on new host CPUs.

QEMU has two operating modes:

• User mode emulation. In this mode, QEMU can launch Linux processes compiled for one CPU on another CPU. Linux system calls are converted because of endianness and 32/64 bit mismatches. The Wine Windows API emulator (http://www.winehq.org) and the DOSEMU DOS emulator (www.dosemu.org) are the main targets for QEMU.

• Full system emulation. In this mode, QEMU emulates a full system, including a processor and various peripherials. Currently, it is only used to launch an x86 Linux kernel on an x86 Linux system. It enables easier testing and debugging of system code. It can also be used to provide virtual hosting of several virtual PCs on a single server.

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

QEMU generic features:

• User space only or full system emulation.

• Using dynamic translation to native code for reasonnable speed.

• Working on x86 and PowerPC hosts. Being tested on ARM, Sparc32, Alpha and S390.

• Self-modifying code support.

• Precise exceptions support.

• The virtual CPU is a library (libqemu) which can be used in other projects.

QEMU user mode emulation features:

• Generic Linux system call converter, including most ioctls.

• clone() emulation using native CPU clone() to use Linux scheduler for threads.

• Accurate signal handling by remapping host signals to target signals.

• QEMU can either use a full software MMU for maximum portability or use the host system call mmap() to simulate the target MMU.

1.2 x86 emulation

QEMU x86 target features:

• The virtual x86 CPU supports 16 bit and 32 bit addressing with segmentation. LDT/GDT and IDT are emulated. VM86 mode is also supported to run DOSEMU.

• Support of host page sizes bigger than 4KB in user mode emulation.

• QEMU can emulate itself on x86.

• An extensive Linux x86 CPU test program is included `tests/test-i386'. It can be used to test other x86 virtual CPUs.

Current QEMU limitations:

• No SSE/MMX support (yet).

• No x86-64 support.

• IPC syscalls are missing.

• The x86 segment limits and access rights are not tested at every memory access.

• On non x86 host CPUs, doubles are used instead of the non standard 10 byte long doubles of x86 for floating point emulation to get maximum performances.

• Some priviledged instructions or behaviors are missing, especially for segment protection testing (yet).

1.3 ARM emulation

• ARM emulation can currently launch small programs while using the generic dynamic code generation architecture of QEMU.

• No FPU support (yet).

• No automatic regression testing (yet).

1.4 SPARC emulation

The SPARC emulation is currently in development.

2. Installation

If you want to compile QEMU, please read the `README' which gives the related information. Otherwise just 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

3. QEMU User space emulator invocation

3.1 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.

• On x86, you can just try to launch any process by using the native libraries:

  qemu-i386 -L / /bin/ls

  -L / tells that the x86 dynamic linker must be searched with a `/' prefix.

• Since QEMU is also a linux process, you can launch qemu with qemu (NOTE: you can only do that if you compiled QEMU from the sources):

  qemu-i386 -L / qemu-i386 -L / /bin/ls

• On non x86 CPUs, you need first to download at least an x86 glibc (`qemu-runtime-i386-XXX-.tar.gz' on the QEMU web page). Ensure that LD_LIBRARY_PATH is not set:

  unset LD_LIBRARY_PATH

  Then you can launch the precompiled `ls' x86 executable:

  qemu-i386 tests/i386/ls

  You can look at `qemu-binfmt-conf.sh' so that QEMU is automatically launched by the Linux kernel when you try to launch x86 executables. It requires the binfmt_misc module in the Linux kernel.

• The x86 version of QEMU is also included. You can try weird things such as:

  qemu-i386 /usr/local/qemu-i386/bin/qemu-i386 /usr/local/qemu-i386/bin/ls-i386

3.2 Wine launch

• Ensure that you have a working QEMU with the x86 glibc distribution (see previous section). In order to verify it, you must be able to do:

  qemu-i386 /usr/local/qemu-i386/bin/ls-i386

• Download the binary x86 Wine install (`qemu-XXX-i386-wine.tar.gz' on the QEMU web page).

• Configure Wine on your account. Look at the provided script `/usr/local/qemu-i386/bin/wine-conf.sh'. Your previous ${HOME}/.wine directory is saved to ${HOME}/.wine.org.

• Then you can try the example `putty.exe':

  qemu-i386 /usr/local/qemu-i386/wine/bin/wine /usr/local/qemu-i386/wine/c/Program\ Files/putty.exe

3.3 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

4. QEMU System emulator invocation

4.1 Introduction

The QEMU System emulator simulates a complete PC. It can either boot directly a Linux kernel (without any BIOS or boot loader) or boot like a real PC with the included BIOS.

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 Linux kernel must be used (See section 4.6 Linux Kernel Compilation).

2. qemu uses a software MMU. It is about two times slower but gives a more accurate emulation.

QEMU emulates the following PC peripherials:

• VGA (hardware level, including all non standard modes)
• PS/2 mouse and keyboard
• IDE disk interface (port=0x1f0, irq=14)
• NE2000 network adapter (port=0x300, irq=9)
• Serial port (port=0x3f8, irq=4)
• PIC (interrupt controler)
• PIT (timers)
• CMOS memory

4.2 Quick Start

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

qemu linux.img

Linux should boot and give you a prompt.

4.3 Direct Linux Boot and Network emulation

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 Uncompressing Linux... Ok, booting the kernel. Linux version 2.4.20 (fabrice@localhost.localdomain) (gcc version 2.96 20000731 (Red Hat Linux 7.3 2.96-110)) #22 lun jui 7 13:37:41 CEST 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 ide1=noprobe ide2=noprobe ide3=noprobe ide4=noprobe ide5=noprobe ide_setup: ide1=noprobe ide_setup: ide2=noprobe ide_setup: ide3=noprobe ide_setup: ide4=noprobe ide_setup: ide5=noprobe Initializing CPU#0 Detected 501.285 MHz processor. Calibrating delay loop... 989.59 BogoMIPS Memory: 29268k/32768k available (907k kernel code, 3112k reserved, 212k data, 52k 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 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 Uniform Multi-Platform E-IDE driver Revision: 6.31 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: 12288 sectors (6 MB) w/256KiB Cache, CHS=12/16/63 Partition check: hda: unknown partition table 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 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: 52k freed sh: can't access tty; job control turned off #

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 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).

4.4 Invocation

usage: qemu [options] [disk_image]

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

General options:

`-hda file'

`-hdb file'

Use file as hard disk 0 or 1 image (See section 4.5 Disk Images).

`-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 4.5 Disk Images).

`-m megs'

Set virtual RAM size to megs megabytes.

`-n script'

Set 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.

`-initrd file'

Use file as initial ram disk.

`-tun-fd fd'

Assumes fd talks to tap/tun and use it. Read http://bellard.org/qemu/tetrinet.html to have an example of its use.

`-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.

Linux boot specific (does not require a full PC boot with a BIOS):

`-kernel bzImage'

Use bzImage as kernel image.

`-append cmdline'

Use cmdline as kernel command line

`-initrd file'

Use file as initial ram disk.

Debug options:

`-s'

Wait gdb connection to port 1234 (See section 4.7 GDB usage).

`-p port'

Change gdb connection port.

`-d'

Output log in /tmp/qemu.log

During emulation, use C-a h to get terminal commands:

C-a h

Print this help

C-a x

Exit emulatior

C-a s

Save disk data back to file (if -snapshot)

C-a b

Send break (magic sysrq)

C-a C-a

Send C-a

4.5 Disk Images

4.5.1 Raw disk images

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

dd if=/dev/zero of=myimage bs=1024 count=mysize

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

4.5.2 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.

4.5.3 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.

4.6 Linux Kernel Compilation

You can use any linux kernel with QEMU. However, if you want to use qemu-fast to get maximum performances, you should make the following changes to the Linux kernel (only 2.4.x and 2.5.x were tested):

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 2.5 kernel as host kernel but if you use a target 2.5 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 < 2.5. 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').

4.7 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 arch/i386/boot/bzImage -hda root-2.4.20.img 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 locahost: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.

5. QEMU Internals

5.1 QEMU compared to other emulators

Like bochs [3], QEMU emulates an x86 CPU. But QEMU is much faster than bochs as it uses dynamic compilation and because it uses the host MMU to simulate the x86 MMU. The downside is that currently the emulation is not as accurate as bochs (for example, you cannot currently run Windows inside QEMU).

Like Valgrind [2], QEMU does user space emulation and dynamic translation. Valgrind is mainly a memory debugger while QEMU has no support for it (QEMU could be used to detect out of bound memory accesses as Valgrind, but it has no support to track uninitialised data as Valgrind does). The Valgrind dynamic translator generates better code than QEMU (in particular it does register allocation) but it is closely tied to an x86 host and target and has no support for precise exceptions and system emulation.

EM86 [4] is the closest project to user space QEMU (and QEMU still uses some of its code, in particular the ELF file loader). EM86 was limited to an alpha host and used a proprietary and slow interpreter (the interpreter part of the FX!32 Digital Win32 code translator [5]).

TWIN [6] is a Windows API emulator like Wine. It is less accurate than Wine but includes a protected mode x86 interpreter to launch x86 Windows executables. Such an approach as greater potential because most of the Windows API is executed natively but it is far more difficult to develop because all the data structures and function parameters exchanged between the API and the x86 code must be converted.

User mode Linux [7] was the only solution before QEMU to launch a Linux kernel as a process while not needing any host kernel patches. However, user mode Linux requires heavy kernel patches while QEMU accepts unpatched Linux kernels. It would be interesting to compare the performance of the two approaches.

The new Plex86 [8] PC virtualizer is done in the same spirit as the QEMU system emulator. It requires a patched Linux kernel to work (you cannot launch the same kernel on your PC), but the patches are really small. As it is a PC virtualizer (no emulation is done except for some priveledged instructions), it has the potential of being faster than QEMU. The downside is that a complicated (and potentially unsafe) host kernel patch is needed.

5.2 Portable dynamic translation

QEMU is a dynamic translator. When it first encounters a piece of code, it converts it to the host instruction set. Usually dynamic translators are very complicated and highly CPU dependent. QEMU uses some tricks which make it relatively easily portable and simple while achieving good performances.

The basic idea is to split every x86 instruction into fewer simpler instructions. Each simple instruction is implemented by a piece of C code (see `op-i386.c'). Then a compile time tool (`dyngen') takes the corresponding object file (`op-i386.o') to generate a dynamic code generator which concatenates the simple instructions to build a function (see `op-i386.h:dyngen_code()').

In essence, the process is similar to [1], but more work is done at compile time.

A key idea to get optimal performances is that constant parameters can be passed to the simple operations. For that purpose, dummy ELF relocations are generated with gcc for each constant parameter. Then, the tool (`dyngen') can locate the relocations and generate the appriopriate C code to resolve them when building the dynamic code.

That way, QEMU is no more difficult to port than a dynamic linker.

To go even faster, GCC static register variables are used to keep the state of the virtual CPU.

5.3 Register allocation

Since QEMU uses fixed simple instructions, no efficient register allocation can be done. However, because RISC CPUs have a lot of register, most of the virtual CPU state can be put in registers without doing complicated register allocation.

5.4 Condition code optimisations

Good CPU condition codes emulation (EFLAGS register on x86) is a critical point to get good performances. QEMU uses lazy condition code evaluation: instead of computing the condition codes after each x86 instruction, it just stores one operand (called CC_SRC), the result (called CC_DST) and the type of operation (called CC_OP).

CC_OP is almost never explicitely set in the generated code because it is known at translation time.

In order to increase performances, a backward pass is performed on the generated simple instructions (see translate-i386.c:optimize_flags()). When it can be proved that the condition codes are not needed by the next instructions, no condition codes are computed at all.

5.5 CPU state optimisations

The x86 CPU has many internal states which change the way it evaluates instructions. In order to achieve a good speed, the translation phase considers that some state information of the virtual x86 CPU cannot change in it. For example, if the SS, DS and ES segments have a zero base, then the translator does not even generate an addition for the segment base.

[The FPU stack pointer register is not handled that way yet].

5.6 Translation cache

A 2MByte cache holds the most recently used translations. For simplicity, it is completely flushed when it is full. A translation unit contains just a single basic block (a block of x86 instructions terminated by a jump or by a virtual CPU state change which the translator cannot deduce statically).

5.7 Direct block chaining

After each translated basic block is executed, QEMU uses the simulated Program Counter (PC) and other cpu state informations (such as the CS segment base value) to find the next basic block.

In order to accelerate the most common cases where the new simulated PC is known, QEMU can patch a basic block so that it jumps directly to the next one.

The most portable code uses an indirect jump. An indirect jump makes it easier to make the jump target modification atomic. On some architectures (such as PowerPC), the JUMP opcode is directly patched so that the block chaining has no overhead.

5.8 Self-modifying code and translated code invalidation

Self-modifying code is a special challenge in x86 emulation because no instruction cache invalidation is signaled by the application when code is modified.

When translated code is generated for a basic block, the corresponding host page is write protected if it is not already read-only (with the system call mprotect()). Then, if a write access is done to the page, Linux raises a SEGV signal. QEMU then invalidates all the translated code in the page and enables write accesses to the page.

Correct translated code invalidation is done efficiently by maintaining a linked list of every translated block contained in a given page. Other linked lists are also maintained to undo direct block chaining.

Although the overhead of doing mprotect() calls is important, most MSDOS programs can be emulated at reasonnable speed with QEMU and DOSEMU.

Note that QEMU also invalidates pages of translated code when it detects that memory mappings are modified with mmap() or munmap().

5.9 Exception support

longjmp() is used when an exception such as division by zero is encountered.

The host SIGSEGV and SIGBUS signal handlers are used to get invalid memory accesses. The exact CPU state can be retrieved because all the x86 registers are stored in fixed host registers. The simulated program counter is found by retranslating the corresponding basic block and by looking where the host program counter was at the exception point.

The virtual CPU cannot retrieve the exact EFLAGS register because in some cases it is not computed because of condition code optimisations. It is not a big concern because the emulated code can still be restarted in any cases.

5.10 Linux system call translation

QEMU includes a generic system call translator for Linux. It means that the parameters of the system calls can be converted to fix the endianness and 32/64 bit issues. The IOCTLs are converted with a generic type description system (see `ioctls.h' and `thunk.c').

QEMU supports host CPUs which have pages bigger than 4KB. It records all the mappings the process does and try to emulated the mmap() system calls in cases where the host mmap() call would fail because of bad page alignment.

5.11 Linux signals

Normal and real-time signals are queued along with their information (siginfo_t) as it is done in the Linux kernel. Then an interrupt request is done to the virtual CPU. When it is interrupted, one queued signal is handled by generating a stack frame in the virtual CPU as the Linux kernel does. The sigreturn() system call is emulated to return from the virtual signal handler.

Some signals (such as SIGALRM) directly come from the host. Other signals are synthetized from the virtual CPU exceptions such as SIGFPE when a division by zero is done (see main.c:cpu_loop()).

The blocked signal mask is still handled by the host Linux kernel so that most signal system calls can be redirected directly to the host Linux kernel. Only the sigaction() and sigreturn() system calls need to be fully emulated (see `signal.c').

5.12 clone() system call and threads

The Linux clone() system call is usually used to create a thread. QEMU uses the host clone() system call so that real host threads are created for each emulated thread. One virtual CPU instance is created for each thread.

The virtual x86 CPU atomic operations are emulated with a global lock so that their semantic is preserved.

Note that currently there are still some locking issues in QEMU. In particular, the translated cache flush is not protected yet against reentrancy.

5.13 Self-virtualization

QEMU was conceived so that ultimately it can emulate itself. Although it is not very useful, it is an important test to show the power of the emulator.

Achieving self-virtualization is not easy because there may be address space conflicts. QEMU solves this problem by being an executable ELF shared object as the ld-linux.so ELF interpreter. That way, it can be relocated at load time.

5.14 MMU emulation

For system emulation, QEMU uses the mmap() system call to emulate the target CPU MMU. It works as long the emulated OS does not use an area reserved by the host OS (such as the area above 0xc0000000 on x86 Linux).

It is planned to add a slower but more precise MMU emulation with a software MMU.

5.15 Bibliography

[1]

http://citeseer.nj.nec.com/piumarta98optimizing.html, Optimizing direct threaded code by selective inlining (1998) by Ian Piumarta, Fabio Riccardi.

[2]

http://developer.kde.org/~sewardj/, Valgrind, an open-source memory debugger for x86-GNU/Linux, by Julian Seward.

[3]

http://bochs.sourceforge.net/, the Bochs IA-32 Emulator Project, by Kevin Lawton et al.

[4]

http://www.cs.rose-hulman.edu/~donaldlf/em86/index.html, the EM86 x86 emulator on Alpha-Linux.

[5]

http://www.usenix.org/publications/library/proceedings/usenix-nt97/full_papers/chernoff/chernoff.pdf, DIGITAL FX!32: Running 32-Bit x86 Applications on Alpha NT, by Anton Chernoff and Ray Hookway.

[6]

http://www.willows.com/, Windows API library emulation from Willows Software.

[7]

http://user-mode-linux.sourceforge.net/, The User-mode Linux Kernel.

[8]

http://www.plex86.org/, The new Plex86 project.

6. Regression Tests

In the directory `tests/', various interesting testing programs are available. There are used for regression testing.

6.1 `test-i386'

This program executes most of the 16 bit and 32 bit x86 instructions and generates a text output. It can be compared with the output obtained with a real CPU or another emulator. The target make test runs this program and a diff on the generated output.

The Linux system call modify_ldt() is used to create x86 selectors to test some 16 bit addressing and 32 bit with segmentation cases.

The Linux system call vm86() is used to test vm86 emulation.

Various exceptions are raised to test most of the x86 user space exception reporting.

6.2 `linux-test'

This program tests various Linux system calls. It is used to verify that the system call parameters are correctly converted between target and host CPUs.

6.3 `hello-i386'

Very simple statically linked x86 program, just to test QEMU during a port to a new host CPU.

6.4 `hello-arm'

Very simple statically linked ARM program, just to test QEMU during a port to a new host CPU.

6.5 `sha1'

It is a simple benchmark. Care must be taken to interpret the results because it mostly tests the ability of the virtual CPU to optimize the rol x86 instruction and the condition code computations.

Table of Contents

1. Introduction

1.1 Features
1.2 x86 emulation
1.3 ARM emulation
1.4 SPARC emulation

2. Installation
3. QEMU User space emulator invocation

3.1 Quick Start
3.2 Wine launch
3.3 Command line options

4. QEMU System emulator invocation

4.1 Introduction
4.2 Quick Start
4.3 Direct Linux Boot and Network emulation
4.4 Invocation
4.5 Disk Images

4.5.1 Raw disk images
4.5.2 Snapshot mode
4.5.3 Copy On Write disk images

4.6 Linux Kernel Compilation
4.7 GDB usage

5. QEMU Internals

5.1 QEMU compared to other emulators
5.2 Portable dynamic translation
5.3 Register allocation
5.4 Condition code optimisations
5.5 CPU state optimisations
5.6 Translation cache
5.7 Direct block chaining
5.8 Self-modifying code and translated code invalidation
5.9 Exception support
5.10 Linux system call translation
5.11 Linux signals
5.12 clone() system call and threads
5.13 Self-virtualization
5.14 MMU emulation
5.15 Bibliography

6. Regression Tests

6.1 `test-i386'
6.2 `linux-test'
6.3 `hello-i386'
6.4 `hello-arm'
6.5 `sha1'

Short Table of Contents

1. Introduction
2. Installation
3. QEMU User space emulator invocation
4. QEMU System emulator invocation
5. QEMU Internals
6. Regression Tests

About this document

• 1. Section One
• 1.1 Subsection One-One
• ...
• 1.2 Subsection One-Two
• 1.2.1 Subsubsection One-Two-One
• 1.2.2 Subsubsection One-Two-Two
• 1.2.3 Subsubsection One-Two-Three     <== Current Position
• 1.2.4 Subsubsection One-Two-Four
• 1.3 Subsection One-Three
• ...
• 1.4 Subsection One-Four