QEMU x86 Emulator Reference Documentation
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:
libqemu) which can be used
in other projects.
Current QEMU Limitations:
doubles are used instead of the non standard
10 byte long doubles of x86 for floating point emulation to get
maximum performances.
In order to launch a Linux process, QEMU needs the process executable itself and all the target (x86) dynamic libraries used by it.
qemu -L / /bin/ls
-L / tells that the x86 dynamic linker must be searched with a
`/' prefix.
LD_LIBRARY_PATH is not set:
unset LD_LIBRARY_PATHThen you can launch the precompiled `ls' x86 executable:
qemu /usr/local/qemu-i386/bin/ls-i386You 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.
qemu /usr/local/qemu-i386/bin/ls-i386
${HOME}/.wine directory is saved to ${HOME}/.wine.org.
qemu /usr/local/qemu-i386/wine/bin/wine /usr/local/qemu-i386/wine/c/Program\ Files/putty.exe
usage: qemu [-h] [-d] [-L path] [-s size] program [arguments...]
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.
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.
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.
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.
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].
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].
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].
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').
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').
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.
In the directory `tests/', various interesting x86 testing programs are available. There are used for regression testing.
Very simple statically linked x86 program, just to test QEMU during a port to a new host CPU.
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.
This program tests various signal cases, including SIGFPE, SIGSEGV and SIGILL.
Tests the clone() system call (basic test).
Tests the glibc threads (more complicated than clone() because signals
are also used).
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.
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).
This document was generated on 30 March 2003 using texi2html 1.56k.