Introduction
************

QEMU is an x86 processor emulator. Its purpose is to run x86 Linux processes on non-x86 Linux architectures such as PowerPC or ARM. By using dynamic translation it achieves a reasonnable speed while being easy to port on new host CPUs. Its main goal is to be able to launch the Wine Windows API emulator (http://www.winehq.org) on non-x86 CPUs.

QEMU features:

  * User space only x86 emulator.

  * Currently ported on i386, PowerPC and S390.

  * Using dynamic translation to native code for reasonnable speed.

  * The virtual x86 CPU supports 16 bit and 32 bit addressing with segmentation. User space LDT and GDT are emulated. VM86 mode is also supported (experimental).

  * 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 virtual x86 signals.

  * QEMU can emulate itself on x86 (experimental).

  * The virtual x86 CPU is a library (libqemu) which can be used in other projects.

  * 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:

  * Not all x86 exceptions are precise (yet). [Very few programs need that].

  * No support for self-modifying code (yet). [Very few programs need that, a notable exception is QEMU itself !].

  * No SSE/MMX support (yet).

  * No x86-64 support.

  * Some Linux syscalls are missing.

  * The x86 segment limits and access rights are not tested at every memory access (and will never be to have good performances).

  * 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.

Invocation
**********

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 -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:

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

  * On non x86 CPUs, you need first to download at least an x86 glibc (qemu-XXX-i386-glibc21.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 /usr/local/qemu-i386/bin/ls-i386

     You can look at /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 binfmt_misc module in the Linux kernel.

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

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

Wine launch (Currently only tested when emulating x86 on x86)
=============================================================

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

Command line options
====================

     usage: qemu [-h] [-d] [-L path] [-s size] program [arguments...]

-h
     Print the help

-d
     Activate log (logfile=/tmp/qemu.log)

-L path
     Set the x86 elf interpreter prefix (default=/usr/local/qemu-i386)

-s size
     Set the x86 stack size in bytes (default=524288)

QEMU Internals
**************

QEMU compared to other emulators
================================

Unlike bochs [3], QEMU emulates only a user space x86 CPU. It means that you cannot launch an operating system with it. The benefit is that it is simpler and faster due to the fact that some of the low level CPU state can be ignored (in particular, no virtual memory needs to be emulated).

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). Valgrind dynamic translator generates better code than QEMU (in particular it does register allocation) but it is closely tied to an x86 host.

EM86 [4] is the closest project to 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.

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 dependant. 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.

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.

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.

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].

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).

[Currently, the translated code is not patched if it jumps to another translated code].

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.

[Currently, the virtual CPU cannot retrieve the exact CPU state in some exceptions, although it could except for the EFLAGS register].

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).

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).

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.

Self-virtualization
===================

QEMU was conceived so that ultimately it can emulate itself. Althought 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.

Since self-modifying code is not supported yet, QEMU cannot emulate itself in case of translation cache flush. This limitation will be suppressed soon.

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.

Regression Tests
****************

In the directory tests/, various interesting x86 testing programs are available. There are used for regression testing.

hello
=====

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

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.

testsig
=======

This program tests various signal cases, including SIGFPE, SIGSEGV and SIGILL.

testclone
=========

Tests the clone() system call (basic test).

testthread
==========

Tests the glibc threads (more complicated than clone() because signals are also used).

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.

runcom
======

A very simple MSDOS emulator to test the Linux vm86() system call emulation. The excellent 54 byte pi_10.com PI number calculator can be launched with it. pi_10.com was written by Bertram Felgenhauer (more information at http://www.boo.net/~jasonp/pipage.html).