Archipelago » qemu

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@settitle QEMU Internals

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@center @titlefont{QEMU Internals}

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@node Top

@top

@menu

* Introduction::

* QEMU Internals::

* Regression Tests::

* Index::

@end menu

@end ifnottex

@contents

@node Introduction

@chapter Introduction

@menu

* intro_features::        Features

* intro_x86_emulation::   x86 and x86-64 emulation

* intro_arm_emulation::   ARM emulation

* intro_mips_emulation::  MIPS emulation

* intro_ppc_emulation::   PowerPC emulation

* intro_sparc_emulation:: Sparc32 and Sparc64 emulation

* intro_other_emulation:: Other CPU emulation

@end menu

@node intro_features

@section Features

QEMU is a FAST! processor emulator using a portable dynamic

translator.

QEMU has two operating modes:

@itemize @minus

@item

Full system emulation. In this mode (full platform virtualization),

QEMU emulates a full system (usually a PC), including a processor and

various peripherals. It can be used to launch several different

Operating Systems at once without rebooting the host machine or to

debug system code.

@item

User mode emulation. In this mode (application level virtualization),

QEMU can launch processes compiled for one CPU on another CPU, however

the Operating Systems must match. This can be used for example to ease

cross-compilation and cross-debugging.

@end itemize

As QEMU requires no host kernel driver 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 reasonable speed.

@item

Working on x86, x86_64 and PowerPC32/64 hosts. Being tested on ARM,

HPPA, Sparc32 and Sparc64. Previous versions had some support for

Alpha and S390 hosts, but TCG (see below) doesn't support those yet.

@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 (look at @file{qemu/tests/qruncom.c} to have an

example of user mode @code{libqemu} usage).

@item

Floating point library supporting both full software emulation and

native host FPU instructions.

@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

Linux user emulator (Linux host only) can be used to launch the Wine

Windows API emulator (@url{http://www.winehq.org}). A Darwin user

emulator (Darwin hosts only) exists and a BSD user emulator for BSD

hosts is under development. It would also be possible to develop a

similar user emulator for Solaris.

QEMU full system emulation features:

@itemize

@item

QEMU uses a full software MMU for maximum portability.

@item

QEMU can optionally use an in-kernel accelerator, like kvm. The accelerators 

execute some of the guest code natively, while

continuing to emulate the rest of the machine.

@item

Various hardware devices can be emulated and in some cases, host

devices (e.g. serial and parallel ports, USB, drives) can be used

transparently by the guest Operating System. Host device passthrough

can be used for talking to external physical peripherals (e.g. a

webcam, modem or tape drive).

@item

Symmetric multiprocessing (SMP) even on a host with a single CPU. On a

SMP host system, QEMU can use only one CPU fully due to difficulty in

implementing atomic memory accesses efficiently.

@end itemize

@node intro_x86_emulation

@section x86 and x86-64 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. There is some support for MMX/3DNow!, SSE, SSE2, SSE3, SSSE3,

and SSE4 as well as x86-64 SVM.

@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 Limited x86-64 support.

@item IPC syscalls are missing.

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

memory access (yet). Hopefully, very few OSes seem to rely on that for

normal use.

@end itemize

@node intro_arm_emulation

@section ARM emulation

@itemize

@item Full ARM 7 user emulation.

@item NWFPE FPU support included in user Linux emulation.

@item Can run most ARM Linux binaries.

@end itemize

@node intro_mips_emulation

@section MIPS emulation

@itemize

@item The system emulation allows full MIPS32/MIPS64 Release 2 emulation,

including privileged instructions, FPU and MMU, in both little and big

endian modes.

@item The Linux userland emulation can run many 32 bit MIPS Linux binaries.

@end itemize

Current QEMU limitations:

@itemize

@item Self-modifying code is not always handled correctly.

@item 64 bit userland emulation is not implemented.

@item The system emulation is not complete enough to run real firmware.

@item The watchpoint debug facility is not implemented.

@end itemize

@node intro_ppc_emulation

@section PowerPC emulation

@itemize

@item Full PowerPC 32 bit emulation, including privileged instructions,

FPU and MMU.

@item Can run most PowerPC Linux binaries.

@end itemize

@node intro_sparc_emulation

@section Sparc32 and Sparc64 emulation

@itemize

@item Full SPARC V8 emulation, including privileged

instructions, FPU and MMU. SPARC V9 emulation includes most privileged

and VIS instructions, FPU and I/D MMU. Alignment is fully enforced.

@item Can run most 32-bit SPARC Linux binaries, SPARC32PLUS Linux binaries and

some 64-bit SPARC Linux binaries.

@end itemize

Current QEMU limitations:

@itemize

@item IPC syscalls are missing.

@item Floating point exception support is buggy.

@item Atomic instructions are not correctly implemented.

@item There are still some problems with Sparc64 emulators.

@end itemize

@node intro_other_emulation

@section Other CPU emulation

In addition to the above, QEMU supports emulation of other CPUs with

varying levels of success. These are:

@itemize

@item

Alpha

@item

CRIS

@item

M68k

@item

SH4

@end itemize

@node QEMU Internals

@chapter QEMU Internals

@menu

* QEMU compared to other emulators::

* Portable dynamic translation::

* Condition code optimisations::

* CPU state optimisations::

* Translation cache::

* Direct block chaining::

* Self-modifying code and translated code invalidation::

* Exception support::

* MMU emulation::

* Device emulation::

* Hardware interrupts::

* User emulation specific details::

* Bibliography::

@end menu

@node QEMU compared to other emulators

@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. Bochs is closely tied to x86 PC

emulation while QEMU can emulate several processors.

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 has 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. The price to pay is that QEMU is

slower.

The Plex86 [8] PC virtualizer is done in the same spirit as the now

obsolete qemu-fast 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 privileged instructions), it has the potential of

being faster than QEMU. The downside is that a complicated (and

potentially unsafe) host kernel patch is needed.

The commercial PC Virtualizers (VMWare [9], VirtualPC [10], TwoOStwo

[11]) are faster than QEMU, but they all need specific, proprietary

and potentially unsafe host drivers. Moreover, they are unable to

provide cycle exact simulation as an emulator can.

VirtualBox [12], Xen [13] and KVM [14] are based on QEMU. QEMU-SystemC

[15] uses QEMU to simulate a system where some hardware devices are

developed in SystemC.

@node Portable dynamic translation

@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.

After the release of version 0.9.1, QEMU switched to a new method of

generating code, Tiny Code Generator or TCG. TCG relaxes the

dependency on the exact version of the compiler used. The basic idea

is to split every target instruction into a couple of RISC-like TCG

ops (see @code{target-i386/translate.c}). Some optimizations can be

performed at this stage, including liveness analysis and trivial

constant expression evaluation. TCG ops are then implemented in the

host CPU back end, also known as TCG target (see

@code{tcg/i386/tcg-target.c}). For more information, please take a

look at @code{tcg/README}.

@node Condition code optimisations

@section Condition code optimisations

Lazy evaluation of CPU condition codes (@code{EFLAGS} register on x86)

is important for CPUs where every instruction sets the condition

codes. It tends to be less important on conventional RISC systems

where condition codes are only updated when explicitly requested. On

Sparc64, costly update of both 32 and 64 bit condition codes can be

avoided with lazy evaluation.

Instead of computing the condition codes after each x86 instruction,

QEMU just stores one operand (called @code{CC_SRC}), the result

(called @code{CC_DST}) and the type of operation (called

@code{CC_OP}). When the condition codes are needed, the condition

codes can be calculated using this information. In addition, an

optimized calculation can be performed for some instruction types like

conditional branches.

@code{CC_OP} is almost never explicitly set in the generated code

because it is known at translation time.

The lazy condition code evaluation is used on x86, m68k, cris and

Sparc. ARM uses a simplified variant for the N and Z flags.

@node CPU state optimisations

@section CPU state optimisations

The target CPUs have 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

CPU cannot change in it. The state is recorded in the Translation

Block (TB). If the state changes (e.g. privilege level), a new TB will

be generated and the previous TB won't be used anymore until the state

matches the state recorded in the previous TB. 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].

@node Translation cache

@section Translation cache

A 16 MByte 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).

@node Direct block chaining

@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 host

architectures (such as x86 or PowerPC), the @code{JUMP} opcode is

directly patched so that the block chaining has no overhead.

@node Self-modifying code and translated code invalidation

@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. 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.

On RISC targets, correctly written software uses memory barriers and

cache flushes, so some of the protection above would not be

necessary. However, QEMU still requires that the generated code always

matches the target instructions in memory in order to handle

exceptions correctly.

@node Exception support

@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 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.

@node MMU emulation

@section MMU emulation

For system emulation QEMU supports a soft MMU. In that mode, the MMU

virtual to physical address translation is done at every memory

access. QEMU uses an address translation cache to speed up the

translation.

In order to avoid flushing the translated code each time the MMU

mappings change, QEMU uses a physically indexed translation cache. It

means that each basic block is indexed with its physical address.

When MMU mappings change, only the chaining of the basic blocks is

reset (i.e. a basic block can no longer jump directly to another one).

@node Device emulation

@section Device emulation

Systems emulated by QEMU are organized by boards. At initialization

phase, each board instantiates a number of CPUs, devices, RAM and

ROM. Each device in turn can assign I/O ports or memory areas (for

MMIO) to its handlers. When the emulation starts, an access to the

ports or MMIO memory areas assigned to the device causes the

corresponding handler to be called.

RAM and ROM are handled more optimally, only the offset to the host

memory needs to be added to the guest address.

The video RAM of VGA and other display cards is special: it can be

read or written directly like RAM, but write accesses cause the memory

to be marked with VGA_DIRTY flag as well.

QEMU supports some device classes like serial and parallel ports, USB,

drives and network devices, by providing APIs for easier connection to

the generic, higher level implementations. The API hides the

implementation details from the devices, like native device use or

advanced block device formats like QCOW.

Usually the devices implement a reset method and register support for

saving and loading of the device state. The devices can also use

timers, especially together with the use of bottom halves (BHs).

@node Hardware interrupts

@section Hardware interrupts

In order to be faster, QEMU does not check at every basic block if an

hardware interrupt is pending. Instead, the user must asynchrously

call a specific function to tell that an interrupt is pending. This

function resets the chaining of the currently executing basic

block. It ensures that the execution will return soon in the main loop

of the CPU emulator. Then the main loop can test if the interrupt is

pending and handle it.

@node User emulation specific details

@section User emulation specific details

@subsection 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.

@subsection 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}).

@subsection 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.

@subsection 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 user emulators solve 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.

@node Bibliography

@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.

@item [9]

@url{http://www.vmware.com/},

The VMWare PC virtualizer.

@item [10]

@url{http://www.microsoft.com/windowsxp/virtualpc/},

The VirtualPC PC virtualizer.

@item [11]

@url{http://www.twoostwo.org/},

The TwoOStwo PC virtualizer.

@item [12]

@url{http://virtualbox.org/},

The VirtualBox PC virtualizer.

@item [13]

@url{http://www.xen.org/},

The Xen hypervisor.

@item [14]

@url{http://kvm.qumranet.com/kvmwiki/Front_Page},

Kernel Based Virtual Machine (KVM).

@item [15]

@url{http://www.greensocs.com/projects/QEMUSystemC},

QEMU-SystemC, a hardware co-simulator.

@end table

@node Regression Tests

@chapter Regression Tests

In the directory @file{tests/}, various interesting testing programs

are available. They are used for regression testing.

@menu

* test-i386::

* linux-test::

* qruncom.c::

@end menu

@node test-i386

@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.

@node linux-test

@section @file{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.

@node qruncom.c

@section @file{qruncom.c}

Example of usage of @code{libqemu} to emulate a user mode i386 CPU.

@node Index

@chapter Index

@printindex cp

@bye