qemu/qemu-doc.texi

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@settitle QEMU x86 Emulator Reference Documentation
@titlepage
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@center @titlefont{QEMU x86 Emulator Reference Documentation}
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@chapter Introduction

QEMU is an x86 processor emulator. Its purpose is to run x86 Linux
processes on non-x86 Linux architectures such as PowerPC. 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
@code{Wine} Windows API emulator (@url{http://www.winehq.org}) or
@code{DOSEMU} (@url{http://www.dosemu.org}) on non-x86 CPUs.

QEMU features:

@itemize 

@item User space only x86 emulator.

@item Currently ported on i386, PowerPC. Work in progress for S390, Alpha and Sparc.

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

@item 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.

@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 virtual x86 signals. 

@item Precise user space x86 exceptions.

@item Self-modifying code support.

@item Support of host page sizes bigger than 4KB.

@item QEMU can emulate itself on x86.

@item The virtual x86 CPU is a library (@code{libqemu}) which can be used 
in other projects.

@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 (and will never be to have good performances).

@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.

@end itemize

@chapter Invocation

@section 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. 

@itemize

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

@example 
qemu -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 -L / qemu -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 /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 /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 /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 /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 [-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 Internals

@section 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.

@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 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 @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. 

Althought 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. 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.

@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.

@end table

@chapter Regression Tests

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

@section @file{hello}

Very simple statically linked x86 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.