[qemu] / qemu-doc.texi

\input texinfo @c -*- texinfo -*-

@iftex
@settitle QEMU CPU Emulator Reference Documentation
@titlepage
@sp 7
@center @titlefont{QEMU CPU Emulator Reference Documentation}
@sp 3
@end titlepage
@end iftex

@chapter Introduction

@section 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:

@itemize @minus

@item 
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 (@url{http://www.winehq.org}) and the DOSEMU DOS emulator
(@url{www.dosemu.org}) are the main targets for QEMU.

@item 
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.

@end itemize

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

QEMU generic features:

@itemize 

@item User space only or full system emulation.

@item Using dynamic translation to native code for reasonnable speed.

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

@item Self-modifying code support.

@item Precise exceptions support.

@item The virtual CPU is a library (@code{libqemu}) which can be used 
in other projects.

@end itemize

QEMU user mode emulation features:
@itemize 
@item Generic Linux system call converter, including most ioctls.

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

@item Accurate signal handling by remapping host signals to target signals. 
@end itemize
@end itemize

QEMU full system emulation features:
@itemize 
@item Using mmap() system calls to simulate the MMU
@end itemize

@section x86 emulation

QEMU x86 target features:

@itemize 

@item 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.

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

@item QEMU can emulate itself on x86.

@item An extensive Linux x86 CPU test program is included @file{tests/test-i386}. 
It can be used to test other x86 virtual CPUs.

@end itemize

Current QEMU limitations:

@itemize 

@item No SSE/MMX support (yet).

@item No x86-64 support.

@item IPC syscalls are missing.

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

@item On non x86 host CPUs, @code{double}s are used instead of the non standard 
10 byte @code{long double}s of x86 for floating point emulation to get
maximum performances.

@item Full system emulation only works if no data are mapped above the virtual address 
0xc0000000 (yet).

@item Some priviledged instructions or behaviors are missing. Only the ones 
needed for proper Linux kernel operation are emulated.

@item No memory separation between the kernel and the user processes is done. 
It will be implemented very soon.

@end itemize

@section ARM emulation

@itemize

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

@item No FPU support (yet).

@item No automatic regression testing (yet).

@end itemize

@section SPARC emulation

The SPARC emulation is currently in development.

@chapter QEMU User space emulator invocation

@section Quick Start

If you need to compile QEMU, please read the @file{README} which gives
the related information.

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

@itemize

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

@example 
qemu-i386 -L / /bin/ls
@end example

@code{-L /} tells that the x86 dynamic linker must be searched with a
@file{/} prefix.

@item Since QEMU is also a linux process, you can launch qemu with qemu:

@example 
qemu-i386 -L / qemu-i386 -L / /bin/ls
@end example

@item On non x86 CPUs, you need first to download at least an x86 glibc
(@file{qemu-XXX-i386-glibc21.tar.gz} on the QEMU web page). Ensure that
@code{LD_LIBRARY_PATH} is not set:

@example
unset LD_LIBRARY_PATH 
@end example

Then you can launch the precompiled @file{ls} x86 executable:

@example
qemu-i386 /usr/local/qemu-i386/bin/ls-i386
@end example
You can look at @file{/usr/local/qemu-i386/bin/qemu-conf.sh} so that
QEMU is automatically launched by the Linux kernel when you try to
launch x86 executables. It requires the @code{binfmt_misc} module in the
Linux kernel.

@item The x86 version of QEMU is also included. You can try weird things such as:
@example
qemu-i386 /usr/local/qemu-i386/bin/qemu-i386 /usr/local/qemu-i386/bin/ls-i386
@end example

@end itemize

@section Wine launch

@itemize

@item 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:

@example
qemu-i386 /usr/local/qemu-i386/bin/ls-i386
@end example

@item Download the binary x86 Wine install
(@file{qemu-XXX-i386-wine.tar.gz} on the QEMU web page). 

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

@item Then you can try the example @file{putty.exe}:

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

@end itemize

@section Command line options

@example
usage: qemu-i386 [-h] [-d] [-L path] [-s size] program [arguments...]
@end example

@table @option
@item -h
Print the help
@item -L path   
Set the x86 elf interpreter prefix (default=/usr/local/qemu-i386)
@item -s size
Set the x86 stack size in bytes (default=524288)
@end table

Debug options:

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

@chapter QEMU System emulator invocation

@section Introduction

@c man begin DESCRIPTION

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:

@enumerate

@item 
@code{qemu} uses the host Memory Management Unit (MMU) to simulate 
the x86 MMU. It is @emph{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 (@xref{linux_compile}).

@item 
@code{qemu-softmmu} uses a software MMU. It is about @emph{two times 
slower} but gives a more accurate emulation. (XXX: Linux cannot be ran
unpatched yet).

@end enumerate

QEMU emulates the following PC peripherials:

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

@c man end

@section Quick Start

Download the linux image (@file{linux.img}) and type:

@example
qemu-softmmu linux.img
@end example

Linux should boot and give you a prompt.

@section 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.

@enumerate
@item
Download the archive @file{linux-test-xxx.tar.gz} containing a Linux
kernel and a disk image. 

@item Optional: If you want network support (for example to launch X11 examples), you
must copy the script @file{qemu-ifup} in @file{/etc} and configure
properly @code{sudo} so that the command @code{ifconfig} contained in
@file{qemu-ifup} can be executed as root. You must verify that your host
kernel supports the TUN/TAP network interfaces: the device
@file{/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.

@item Launch @code{qemu.sh}. You should have the following output:

@example
> ./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
#
@end example

@item
Then you can play with the kernel inside the virtual serial console. You
can launch @code{ls} for example. Type @key{Ctrl-a h} to have an help
about the keys you can type inside the virtual serial console. In
particular, use @key{Ctrl-a x} to exit QEMU and use @key{Ctrl-a b} as
the Magic SysRq key.

@item 
If the network is enabled, launch the script @file{/etc/linuxrc} in the
emulator (don't forget the leading dot):
@example
. /etc/linuxrc
@end example

Then enable X11 connections on your PC from the emulated Linux: 
@example
xhost +172.20.0.2
@end example

You can now launch @file{xterm} or @file{xlogo} and verify that you have
a real Virtual Linux system !

@end enumerate

NOTES:
@enumerate
@item 
A 2.5.74 kernel is also included in the archive. Just
replace the bzImage in qemu.sh to try it.

@item 
vl creates a temporary file in @var{$QEMU_TMPDIR} (@file{/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.

@item 
In order to exit cleanly for vl, you can do a @emph{shutdown} inside
vl. vl will automatically exit when the Linux shutdown is done.

@item 
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:
@example
ide1=noprobe ide2=noprobe ide3=noprobe ide4=noprobe ide5=noprobe
@end example

@item 
The example disk image is a modified version of the one made by Kevin
Lawton for the plex86 Project (@url{www.plex86.org}).

@end enumerate

@section Invocation

@example
@c man begin SYNOPSIS
usage: qemu [options] [disk_image]
@c man end
@end example

@c man begin OPTIONS
@var{disk_image} is a raw hard image image for IDE hard disk 0.

General options:
@table @option
@item -hda file
@item -hdb file
Use @var{file} as hard disk 0 or 1 image (@xref{disk_images}). 

@item -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 @key{C-a s} (@xref{disk_images}). 

@item -m megs
Set virtual RAM size to @var{megs} megabytes.

@item -n script      
Set network init script [default=/etc/vl-ifup]. This script is
launched to configure the host network interface (usually tun0)
corresponding to the virtual NE2000 card.

@item -initrd file
Use @var{file} as initial ram disk.

@item -tun-fd fd      
Assumes @var{fd} talks to tap/tun and use it. Read
@url{http://bellard.org/qemu/tetrinet.html} to have an example of its
use.

@item -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.

@end table

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

@item -kernel bzImage 
Use @var{bzImage} as kernel image.

@item -append cmdline 
Use @var{cmdline} as kernel command line

@item -initrd file
Use @var{file} as initial ram disk.

@end table

Debug options:
@table @option
@item -s
Wait gdb connection to port 1234 (@xref{gdb_usage}). 
@item -p port
Change gdb connection port.
@item -d             
Output log in /tmp/vl.log
@end table

During emulation, use @key{C-a h} to get terminal commands:

@table @key
@item C-a h
Print this help
@item C-a x    
Exit emulatior
@item C-a s    
Save disk data back to file (if -snapshot)
@item C-a b
Send break (magic sysrq)
@item C-a C-a
Send C-a
@end table
@c man end

@ignore

@setfilename qemu 
@settitle QEMU System Emulator

@c man begin SEEALSO
The HTML documentation of QEMU for more precise information and Linux
user mode emulator invocation.
@c man end

@c man begin AUTHOR
Fabrice Bellard
@c man end

@end ignore

@end ignore
@node disk_images
@section Disk Images

@subsection Raw disk images

The disk images can simply be raw images of the hard disk. You can
create them with the command:
@example
dd if=/dev/zero of=myimage bs=1024 count=mysize
@end example
where @var{myimage} is the image filename and @var{mysize} is its size
in kilobytes.

@subsection Snapshot mode

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

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

@subsection Copy On Write disk images

QEMU also supports user mode Linux
(@url{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:

@example
qemu-mkcow -f myrawimage.bin mycowimage.cow
@end example

@file{myrawimage.bin} is a raw image you want to use as original disk
image. It will never be written to.

@file{mycowimage.cow} is the COW disk image which is created by
@code{qemu-mkcow}. You can use it directly with the @option{-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 @key{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:

@example
qemu-mkcow mycowimage.cow 1024
@end example

to create a 1 gigabyte empty COW disk image.

NOTES: 
@enumerate
@item
COW disk images must be created on file systems supporting
@emph{holes} such as ext2 or ext3.
@item 
Since holes are used, the displayed size of the COW disk image is not
the real one. To know it, use the @code{ls -ls} command.
@end enumerate

@node linux_compile
@section Linux Kernel Compilation

You should be able to use any kernel with QEMU provided you make the
following changes (only 2.4.x and 2.5.x were tested):

@enumerate
@item
The kernel must be mapped at 0x90000000 (the default is
0xc0000000). You must modify only two lines in the kernel source:

In @file{include/asm/page.h}, replace
@example
#define __PAGE_OFFSET           (0xc0000000)
@end example
by
@example
#define __PAGE_OFFSET           (0x90000000)
@end example

And in @file{arch/i386/vmlinux.lds}, replace
@example
  . = 0xc0000000 + 0x100000;
@end example
by 
@example
  . = 0x90000000 + 0x100000;
@end example

@item
If you want to enable SMP (Symmetric Multi-Processing) support, you
must make the following change in @file{include/asm/fixmap.h}. Replace
@example
#define FIXADDR_TOP     (0xffffX000UL)
@end example
by 
@example
#define FIXADDR_TOP     (0xa7ffX000UL)
@end example
(X is 'e' or 'f' depending on the kernel version). Although you can
use an SMP kernel with QEMU, it only supports one CPU.

@item
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
@file{include/asm/param.h}, replace:

@example
# define HZ             1000            /* Internal kernel timer frequency */
@end example
by
@example
# define HZ             100             /* Internal kernel timer frequency */
@end example

@end enumerate

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

Just type
@example
make bzImage
@end example

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
@file{arch/i386/boot/bzImage}).

@node gdb_usage
@section 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 vl with the '-s' option. It will wait for a
gdb connection:
@example
> vl -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
@end example

Then launch gdb on the 'vmlinux' executable:
@example
> gdb vmlinux
@end example

In gdb, connect to QEMU:
@example
(gdb) target remote locahost:1234
@end example

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

WARNING: breakpoints and single stepping are not yet supported.

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

@enumerate
@item
Use @code{info reg} to display all the CPU registers.
@item
Use @code{x/10i $eip} to display the code at the PC position.
@item
Use @code{set architecture i8086} to dump 16 bit code. Then use
@code{x/10i $cs*16+*eip} to dump the code at the PC position.
@end enumerate

@chapter QEMU Internals

@section 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.

@section 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 @file{op-i386.c}). Then a compile time tool (@file{dyngen})
takes the corresponding object file (@file{op-i386.o}) to generate a
dynamic code generator which concatenates the simple instructions to
build a function (see @file{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 (@file{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.

@section 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.

@section Condition code optimisations

Good CPU condition codes emulation (@code{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 @code{CC_SRC}), the
result (called @code{CC_DST}) and the type of operation (called
@code{CC_OP}).

@code{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
@code{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.

@section 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].

@section 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).

@section 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 @code{JUMP} opcode is directly
patched so that the block chaining has no overhead.

@section 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 @code{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 @code{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 @code{mmap()} or @code{munmap()}.

@section 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 @code{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.

@section 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 @file{ioctls.h} and @file{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 @code{mmap()}
system calls in cases where the host @code{mmap()} call would fail
because of bad page alignment.

@section Linux signals

Normal and real-time signals are queued along with their information
(@code{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 @code{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 @code{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 @code{sigaction()} and @code{sigreturn()} system
calls need to be fully emulated (see @file{signal.c}).

@section 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.

@section 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.

@section 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.

@section Bibliography

@table @asis

@item [1] 
@url{http://citeseer.nj.nec.com/piumarta98optimizing.html}, Optimizing
direct threaded code by selective inlining (1998) by Ian Piumarta, Fabio
Riccardi.

@item [2]
@url{http://developer.kde.org/~sewardj/}, Valgrind, an open-source
memory debugger for x86-GNU/Linux, by Julian Seward.

@item [3]
@url{http://bochs.sourceforge.net/}, the Bochs IA-32 Emulator Project,
by Kevin Lawton et al.

@item [4]
@url{http://www.cs.rose-hulman.edu/~donaldlf/em86/index.html}, the EM86
x86 emulator on Alpha-Linux.

@item [5]
@url{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.

@item [6]
@url{http://www.willows.com/}, Windows API library emulation from
Willows Software.

@item [7]
@url{http://user-mode-linux.sourceforge.net/}, 
The User-mode Linux Kernel.

@item [8]
@url{http://www.plex86.org/}, 
The new Plex86 project.

@end table

@chapter Regression Tests

In the directory @file{tests/}, various interesting testing programs
are available. There are used for regression testing.

@section @file{hello-i386}

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

@section @file{hello-arm}

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

@section @file{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 @code{make test} runs this
program and a @code{diff} on the generated output.

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

The Linux system call @code{vm86()} is used to test vm86 emulation.

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

@section @file{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
@code{rol} x86 instruction and the condition code computations.