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\input texinfo @c -*- texinfo -*- |
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@c %**start of header |
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@setfilename qemu-tech.info |
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@settitle QEMU Internals |
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@exampleindent 0 |
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@paragraphindent 0 |
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@c %**end of header |
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|
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@iftex |
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@titlepage |
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@sp 7 |
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@center @titlefont{QEMU Internals} |
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@sp 3 |
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@end titlepage |
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@end iftex |
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|
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@ifnottex |
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@node Top |
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@top |
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|
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@menu |
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* Introduction:: |
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* QEMU Internals:: |
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* Regression Tests:: |
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* Index:: |
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@end menu |
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@end ifnottex |
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|
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@contents |
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|
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@node Introduction |
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@chapter Introduction |
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|
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@menu |
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* intro_features:: Features |
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* intro_x86_emulation:: x86 and x86-64 emulation |
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* intro_arm_emulation:: ARM emulation |
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* intro_mips_emulation:: MIPS emulation |
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* intro_ppc_emulation:: PowerPC emulation |
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* intro_sparc_emulation:: Sparc32 and Sparc64 emulation |
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* intro_other_emulation:: Other CPU emulation |
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@end menu |
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|
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@node intro_features |
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@section Features |
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|
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QEMU is a FAST! processor emulator using a portable dynamic |
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translator. |
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|
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QEMU has two operating modes: |
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|
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@itemize @minus |
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|
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@item |
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Full system emulation. In this mode (full platform virtualization), |
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QEMU emulates a full system (usually a PC), including a processor and |
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various peripherals. It can be used to launch several different |
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Operating Systems at once without rebooting the host machine or to |
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debug system code. |
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|
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@item |
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User mode emulation. In this mode (application level virtualization), |
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QEMU can launch processes compiled for one CPU on another CPU, however |
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the Operating Systems must match. This can be used for example to ease |
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cross-compilation and cross-debugging. |
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@end itemize |
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|
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As QEMU requires no host kernel driver to run, it is very safe and |
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easy to use. |
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|
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QEMU generic features: |
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|
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@itemize |
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|
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@item User space only or full system emulation. |
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|
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@item Using dynamic translation to native code for reasonable speed. |
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|
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@item |
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Working on x86, x86_64 and PowerPC32/64 hosts. Being tested on ARM, |
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HPPA, Sparc32 and Sparc64. Previous versions had some support for |
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Alpha and S390 hosts, but TCG (see below) doesn't support those yet. |
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|
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@item Self-modifying code support. |
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|
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@item Precise exceptions support. |
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|
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@item The virtual CPU is a library (@code{libqemu}) which can be used |
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in other projects (look at @file{qemu/tests/qruncom.c} to have an |
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example of user mode @code{libqemu} usage). |
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|
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@item |
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Floating point library supporting both full software emulation and |
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native host FPU instructions. |
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|
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@end itemize |
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|
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QEMU user mode emulation features: |
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@itemize |
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@item Generic Linux system call converter, including most ioctls. |
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|
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@item clone() emulation using native CPU clone() to use Linux scheduler for threads. |
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|
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@item Accurate signal handling by remapping host signals to target signals. |
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@end itemize |
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|
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Linux user emulator (Linux host only) can be used to launch the Wine |
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Windows API emulator (@url{http://www.winehq.org}). A Darwin user |
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emulator (Darwin hosts only) exists and a BSD user emulator for BSD |
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hosts is under development. It would also be possible to develop a |
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similar user emulator for Solaris. |
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|
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QEMU full system emulation features: |
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@itemize |
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@item |
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QEMU uses a full software MMU for maximum portability. |
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|
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@item |
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QEMU can optionally use an in-kernel accelerator, like kqemu and |
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kvm. The accelerators execute some of the guest code natively, while |
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continuing to emulate the rest of the machine. |
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|
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@item |
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Various hardware devices can be emulated and in some cases, host |
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devices (e.g. serial and parallel ports, USB, drives) can be used |
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transparently by the guest Operating System. Host device passthrough |
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can be used for talking to external physical peripherals (e.g. a |
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webcam, modem or tape drive). |
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|
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@item |
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Symmetric multiprocessing (SMP) even on a host with a single CPU. On a |
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SMP host system, QEMU can use only one CPU fully due to difficulty in |
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implementing atomic memory accesses efficiently. |
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|
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@end itemize |
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|
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@node intro_x86_emulation |
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@section x86 and x86-64 emulation |
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|
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QEMU x86 target features: |
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|
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@itemize |
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|
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@item The virtual x86 CPU supports 16 bit and 32 bit addressing with segmentation. |
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LDT/GDT and IDT are emulated. VM86 mode is also supported to run |
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DOSEMU. There is some support for MMX/3DNow!, SSE, SSE2, SSE3, SSSE3, |
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and SSE4 as well as x86-64 SVM. |
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|
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@item Support of host page sizes bigger than 4KB in user mode emulation. |
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|
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@item QEMU can emulate itself on x86. |
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|
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@item An extensive Linux x86 CPU test program is included @file{tests/test-i386}. |
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It can be used to test other x86 virtual CPUs. |
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|
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@end itemize |
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|
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Current QEMU limitations: |
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|
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@itemize |
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|
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@item Limited x86-64 support. |
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|
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@item IPC syscalls are missing. |
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|
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@item The x86 segment limits and access rights are not tested at every |
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memory access (yet). Hopefully, very few OSes seem to rely on that for |
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normal use. |
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|
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@end itemize |
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|
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@node intro_arm_emulation |
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@section ARM emulation |
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|
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@itemize |
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|
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@item Full ARM 7 user emulation. |
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|
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@item NWFPE FPU support included in user Linux emulation. |
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|
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@item Can run most ARM Linux binaries. |
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|
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@end itemize |
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|
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@node intro_mips_emulation |
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@section MIPS emulation |
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|
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@itemize |
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|
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@item The system emulation allows full MIPS32/MIPS64 Release 2 emulation, |
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including privileged instructions, FPU and MMU, in both little and big |
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endian modes. |
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|
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@item The Linux userland emulation can run many 32 bit MIPS Linux binaries. |
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|
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@end itemize |
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|
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Current QEMU limitations: |
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|
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@itemize |
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|
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@item Self-modifying code is not always handled correctly. |
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|
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@item 64 bit userland emulation is not implemented. |
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|
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@item The system emulation is not complete enough to run real firmware. |
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|
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@item The watchpoint debug facility is not implemented. |
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|
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@end itemize |
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|
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@node intro_ppc_emulation |
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@section PowerPC emulation |
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|
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@itemize |
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|
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@item Full PowerPC 32 bit emulation, including privileged instructions, |
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FPU and MMU. |
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|
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@item Can run most PowerPC Linux binaries. |
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|
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@end itemize |
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|
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@node intro_sparc_emulation |
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@section Sparc32 and Sparc64 emulation |
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|
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@itemize |
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|
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@item Full SPARC V8 emulation, including privileged |
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instructions, FPU and MMU. SPARC V9 emulation includes most privileged |
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and VIS instructions, FPU and I/D MMU. Alignment is fully enforced. |
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|
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@item Can run most 32-bit SPARC Linux binaries, SPARC32PLUS Linux binaries and |
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some 64-bit SPARC Linux binaries. |
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|
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@end itemize |
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|
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Current QEMU limitations: |
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|
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@itemize |
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|
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@item IPC syscalls are missing. |
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|
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@item Floating point exception support is buggy. |
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|
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@item Atomic instructions are not correctly implemented. |
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|
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@item There are still some problems with Sparc64 emulators. |
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|
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@end itemize |
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|
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@node intro_other_emulation |
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@section Other CPU emulation |
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|
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In addition to the above, QEMU supports emulation of other CPUs with |
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varying levels of success. These are: |
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|
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@itemize |
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|
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@item |
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Alpha |
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@item |
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CRIS |
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@item |
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M68k |
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@item |
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SH4 |
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@end itemize |
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|
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@node QEMU Internals |
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@chapter QEMU Internals |
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|
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@menu |
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* QEMU compared to other emulators:: |
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* Portable dynamic translation:: |
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* Condition code optimisations:: |
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* CPU state optimisations:: |
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* Translation cache:: |
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* Direct block chaining:: |
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* Self-modifying code and translated code invalidation:: |
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* Exception support:: |
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* MMU emulation:: |
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* Device emulation:: |
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* Hardware interrupts:: |
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* User emulation specific details:: |
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* Bibliography:: |
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@end menu |
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|
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@node QEMU compared to other emulators |
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@section QEMU compared to other emulators |
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|
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Like bochs [3], QEMU emulates an x86 CPU. But QEMU is much faster than |
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bochs as it uses dynamic compilation. Bochs is closely tied to x86 PC |
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emulation while QEMU can emulate several processors. |
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|
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Like Valgrind [2], QEMU does user space emulation and dynamic |
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translation. Valgrind is mainly a memory debugger while QEMU has no |
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support for it (QEMU could be used to detect out of bound memory |
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accesses as Valgrind, but it has no support to track uninitialised data |
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as Valgrind does). The Valgrind dynamic translator generates better code |
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than QEMU (in particular it does register allocation) but it is closely |
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tied to an x86 host and target and has no support for precise exceptions |
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and system emulation. |
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|
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EM86 [4] is the closest project to user space QEMU (and QEMU still uses |
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some of its code, in particular the ELF file loader). EM86 was limited |
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to an alpha host and used a proprietary and slow interpreter (the |
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interpreter part of the FX!32 Digital Win32 code translator [5]). |
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|
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TWIN [6] is a Windows API emulator like Wine. It is less accurate than |
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Wine but includes a protected mode x86 interpreter to launch x86 Windows |
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executables. Such an approach has greater potential because most of the |
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Windows API is executed natively but it is far more difficult to develop |
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because all the data structures and function parameters exchanged |
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between the API and the x86 code must be converted. |
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|
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User mode Linux [7] was the only solution before QEMU to launch a |
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Linux kernel as a process while not needing any host kernel |
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patches. However, user mode Linux requires heavy kernel patches while |
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QEMU accepts unpatched Linux kernels. The price to pay is that QEMU is |
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slower. |
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|
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The Plex86 [8] PC virtualizer is done in the same spirit as the now |
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obsolete qemu-fast system emulator. It requires a patched Linux kernel |
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to work (you cannot launch the same kernel on your PC), but the |
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patches are really small. As it is a PC virtualizer (no emulation is |
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done except for some privileged instructions), it has the potential of |
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being faster than QEMU. The downside is that a complicated (and |
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potentially unsafe) host kernel patch is needed. |
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|
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The commercial PC Virtualizers (VMWare [9], VirtualPC [10], TwoOStwo |
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[11]) are faster than QEMU, but they all need specific, proprietary |
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and potentially unsafe host drivers. Moreover, they are unable to |
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provide cycle exact simulation as an emulator can. |
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|
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VirtualBox [12], Xen [13] and KVM [14] are based on QEMU. QEMU-SystemC |
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[15] uses QEMU to simulate a system where some hardware devices are |
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developed in SystemC. |
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|
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@node Portable dynamic translation |
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@section Portable dynamic translation |
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|
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QEMU is a dynamic translator. When it first encounters a piece of code, |
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it converts it to the host instruction set. Usually dynamic translators |
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are very complicated and highly CPU dependent. QEMU uses some tricks |
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which make it relatively easily portable and simple while achieving good |
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performances. |
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|
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After the release of version 0.9.1, QEMU switched to a new method of |
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generating code, Tiny Code Generator or TCG. TCG relaxes the |
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dependency on the exact version of the compiler used. The basic idea |
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is to split every target instruction into a couple of RISC-like TCG |
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ops (see @code{target-i386/translate.c}). Some optimizations can be |
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performed at this stage, including liveness analysis and trivial |
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constant expression evaluation. TCG ops are then implemented in the |
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host CPU back end, also known as TCG target (see |
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@code{tcg/i386/tcg-target.c}). For more information, please take a |
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look at @code{tcg/README}. |
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|
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@node Condition code optimisations |
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@section Condition code optimisations |
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|
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Lazy evaluation of CPU condition codes (@code{EFLAGS} register on x86) |
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is important for CPUs where every instruction sets the condition |
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codes. It tends to be less important on conventional RISC systems |
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where condition codes are only updated when explicitly requested. On |
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Sparc64, costly update of both 32 and 64 bit condition codes can be |
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avoided with lazy evaluation. |
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|
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Instead of computing the condition codes after each x86 instruction, |
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QEMU just stores one operand (called @code{CC_SRC}), the result |
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(called @code{CC_DST}) and the type of operation (called |
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@code{CC_OP}). When the condition codes are needed, the condition |
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codes can be calculated using this information. In addition, an |
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optimized calculation can be performed for some instruction types like |
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conditional branches. |
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|
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@code{CC_OP} is almost never explicitly set in the generated code |
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because it is known at translation time. |
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|
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The lazy condition code evaluation is used on x86, m68k, cris and |
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Sparc. ARM uses a simplified variant for the N and Z flags. |
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|
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@node CPU state optimisations |
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@section CPU state optimisations |
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|
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The target CPUs have many internal states which change the way it |
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evaluates instructions. In order to achieve a good speed, the |
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translation phase considers that some state information of the virtual |
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CPU cannot change in it. The state is recorded in the Translation |
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Block (TB). If the state changes (e.g. privilege level), a new TB will |
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be generated and the previous TB won't be used anymore until the state |
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matches the state recorded in the previous TB. For example, if the SS, |
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DS and ES segments have a zero base, then the translator does not even |
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generate an addition for the segment base. |
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|
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[The FPU stack pointer register is not handled that way yet]. |
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|
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@node Translation cache |
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@section Translation cache |
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|
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A 16 MByte cache holds the most recently used translations. For |
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simplicity, it is completely flushed when it is full. A translation unit |
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contains just a single basic block (a block of x86 instructions |
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terminated by a jump or by a virtual CPU state change which the |
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translator cannot deduce statically). |
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|
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@node Direct block chaining |
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@section Direct block chaining |
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|
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After each translated basic block is executed, QEMU uses the simulated |
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Program Counter (PC) and other cpu state informations (such as the CS |
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segment base value) to find the next basic block. |
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|
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In order to accelerate the most common cases where the new simulated PC |
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is known, QEMU can patch a basic block so that it jumps directly to the |
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next one. |
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|
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The most portable code uses an indirect jump. An indirect jump makes |
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it easier to make the jump target modification atomic. On some host |
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architectures (such as x86 or PowerPC), the @code{JUMP} opcode is |
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directly patched so that the block chaining has no overhead. |
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|
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@node Self-modifying code and translated code invalidation |
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@section Self-modifying code and translated code invalidation |
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|
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Self-modifying code is a special challenge in x86 emulation because no |
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instruction cache invalidation is signaled by the application when code |
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is modified. |
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|
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When translated code is generated for a basic block, the corresponding |
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host page is write protected if it is not already read-only. Then, if |
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a write access is done to the page, Linux raises a SEGV signal. QEMU |
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then invalidates all the translated code in the page and enables write |
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accesses to the page. |
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|
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Correct translated code invalidation is done efficiently by maintaining |
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a linked list of every translated block contained in a given page. Other |
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linked lists are also maintained to undo direct block chaining. |
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|
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On RISC targets, correctly written software uses memory barriers and |
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cache flushes, so some of the protection above would not be |
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necessary. However, QEMU still requires that the generated code always |
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matches the target instructions in memory in order to handle |
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exceptions correctly. |
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|
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@node Exception support |
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@section Exception support |
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|
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longjmp() is used when an exception such as division by zero is |
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encountered. |
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|
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The host SIGSEGV and SIGBUS signal handlers are used to get invalid |
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memory accesses. The simulated program counter is found by |
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retranslating the corresponding basic block and by looking where the |
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host program counter was at the exception point. |
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|
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The virtual CPU cannot retrieve the exact @code{EFLAGS} register because |
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in some cases it is not computed because of condition code |
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optimisations. It is not a big concern because the emulated code can |
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still be restarted in any cases. |
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|
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@node MMU emulation |
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@section MMU emulation |
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|
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For system emulation QEMU supports a soft MMU. In that mode, the MMU |
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virtual to physical address translation is done at every memory |
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access. QEMU uses an address translation cache to speed up the |
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translation. |
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|
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In order to avoid flushing the translated code each time the MMU |
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mappings change, QEMU uses a physically indexed translation cache. It |
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means that each basic block is indexed with its physical address. |
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|
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When MMU mappings change, only the chaining of the basic blocks is |
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reset (i.e. a basic block can no longer jump directly to another one). |
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|
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@node Device emulation |
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@section Device emulation |
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|
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Systems emulated by QEMU are organized by boards. At initialization |
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phase, each board instantiates a number of CPUs, devices, RAM and |
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ROM. Each device in turn can assign I/O ports or memory areas (for |
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MMIO) to its handlers. When the emulation starts, an access to the |
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ports or MMIO memory areas assigned to the device causes the |
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corresponding handler to be called. |
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|
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RAM and ROM are handled more optimally, only the offset to the host |
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memory needs to be added to the guest address. |
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|
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The video RAM of VGA and other display cards is special: it can be |
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read or written directly like RAM, but write accesses cause the memory |
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to be marked with VGA_DIRTY flag as well. |
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|
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QEMU supports some device classes like serial and parallel ports, USB, |
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drives and network devices, by providing APIs for easier connection to |
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the generic, higher level implementations. The API hides the |
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implementation details from the devices, like native device use or |
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advanced block device formats like QCOW. |
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|
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Usually the devices implement a reset method and register support for |
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saving and loading of the device state. The devices can also use |
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timers, especially together with the use of bottom halves (BHs). |
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|
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@node Hardware interrupts |
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@section Hardware interrupts |
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|
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In order to be faster, QEMU does not check at every basic block if an |
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hardware interrupt is pending. Instead, the user must asynchrously |
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call a specific function to tell that an interrupt is pending. This |
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function resets the chaining of the currently executing basic |
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block. It ensures that the execution will return soon in the main loop |
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of the CPU emulator. Then the main loop can test if the interrupt is |
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pending and handle it. |
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|
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@node User emulation specific details |
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@section User emulation specific details |
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|
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@subsection Linux system call translation |
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|
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QEMU includes a generic system call translator for Linux. It means that |
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the parameters of the system calls can be converted to fix the |
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endianness and 32/64 bit issues. The IOCTLs are converted with a generic |
524 |
type description system (see @file{ioctls.h} and @file{thunk.c}). |
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|
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QEMU supports host CPUs which have pages bigger than 4KB. It records all |
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the mappings the process does and try to emulated the @code{mmap()} |
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system calls in cases where the host @code{mmap()} call would fail |
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because of bad page alignment. |
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|
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@subsection Linux signals |
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|
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Normal and real-time signals are queued along with their information |
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(@code{siginfo_t}) as it is done in the Linux kernel. Then an interrupt |
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request is done to the virtual CPU. When it is interrupted, one queued |
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signal is handled by generating a stack frame in the virtual CPU as the |
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Linux kernel does. The @code{sigreturn()} system call is emulated to return |
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from the virtual signal handler. |
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|
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Some signals (such as SIGALRM) directly come from the host. Other |
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signals are synthetized from the virtual CPU exceptions such as SIGFPE |
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when a division by zero is done (see @code{main.c:cpu_loop()}). |
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|
544 |
The blocked signal mask is still handled by the host Linux kernel so |
545 |
that most signal system calls can be redirected directly to the host |
546 |
Linux kernel. Only the @code{sigaction()} and @code{sigreturn()} system |
547 |
calls need to be fully emulated (see @file{signal.c}). |
548 |
|
549 |
@subsection clone() system call and threads |
550 |
|
551 |
The Linux clone() system call is usually used to create a thread. QEMU |
552 |
uses the host clone() system call so that real host threads are created |
553 |
for each emulated thread. One virtual CPU instance is created for each |
554 |
thread. |
555 |
|
556 |
The virtual x86 CPU atomic operations are emulated with a global lock so |
557 |
that their semantic is preserved. |
558 |
|
559 |
Note that currently there are still some locking issues in QEMU. In |
560 |
particular, the translated cache flush is not protected yet against |
561 |
reentrancy. |
562 |
|
563 |
@subsection Self-virtualization |
564 |
|
565 |
QEMU was conceived so that ultimately it can emulate itself. Although |
566 |
it is not very useful, it is an important test to show the power of the |
567 |
emulator. |
568 |
|
569 |
Achieving self-virtualization is not easy because there may be address |
570 |
space conflicts. QEMU user emulators solve this problem by being an |
571 |
executable ELF shared object as the ld-linux.so ELF interpreter. That |
572 |
way, it can be relocated at load time. |
573 |
|
574 |
@node Bibliography |
575 |
@section Bibliography |
576 |
|
577 |
@table @asis |
578 |
|
579 |
@item [1] |
580 |
@url{http://citeseer.nj.nec.com/piumarta98optimizing.html}, Optimizing |
581 |
direct threaded code by selective inlining (1998) by Ian Piumarta, Fabio |
582 |
Riccardi. |
583 |
|
584 |
@item [2] |
585 |
@url{http://developer.kde.org/~sewardj/}, Valgrind, an open-source |
586 |
memory debugger for x86-GNU/Linux, by Julian Seward. |
587 |
|
588 |
@item [3] |
589 |
@url{http://bochs.sourceforge.net/}, the Bochs IA-32 Emulator Project, |
590 |
by Kevin Lawton et al. |
591 |
|
592 |
@item [4] |
593 |
@url{http://www.cs.rose-hulman.edu/~donaldlf/em86/index.html}, the EM86 |
594 |
x86 emulator on Alpha-Linux. |
595 |
|
596 |
@item [5] |
597 |
@url{http://www.usenix.org/publications/library/proceedings/usenix-nt97/@/full_papers/chernoff/chernoff.pdf}, |
598 |
DIGITAL FX!32: Running 32-Bit x86 Applications on Alpha NT, by Anton |
599 |
Chernoff and Ray Hookway. |
600 |
|
601 |
@item [6] |
602 |
@url{http://www.willows.com/}, Windows API library emulation from |
603 |
Willows Software. |
604 |
|
605 |
@item [7] |
606 |
@url{http://user-mode-linux.sourceforge.net/}, |
607 |
The User-mode Linux Kernel. |
608 |
|
609 |
@item [8] |
610 |
@url{http://www.plex86.org/}, |
611 |
The new Plex86 project. |
612 |
|
613 |
@item [9] |
614 |
@url{http://www.vmware.com/}, |
615 |
The VMWare PC virtualizer. |
616 |
|
617 |
@item [10] |
618 |
@url{http://www.microsoft.com/windowsxp/virtualpc/}, |
619 |
The VirtualPC PC virtualizer. |
620 |
|
621 |
@item [11] |
622 |
@url{http://www.twoostwo.org/}, |
623 |
The TwoOStwo PC virtualizer. |
624 |
|
625 |
@item [12] |
626 |
@url{http://virtualbox.org/}, |
627 |
The VirtualBox PC virtualizer. |
628 |
|
629 |
@item [13] |
630 |
@url{http://www.xen.org/}, |
631 |
The Xen hypervisor. |
632 |
|
633 |
@item [14] |
634 |
@url{http://kvm.qumranet.com/kvmwiki/Front_Page}, |
635 |
Kernel Based Virtual Machine (KVM). |
636 |
|
637 |
@item [15] |
638 |
@url{http://www.greensocs.com/projects/QEMUSystemC}, |
639 |
QEMU-SystemC, a hardware co-simulator. |
640 |
|
641 |
@end table |
642 |
|
643 |
@node Regression Tests |
644 |
@chapter Regression Tests |
645 |
|
646 |
In the directory @file{tests/}, various interesting testing programs |
647 |
are available. They are used for regression testing. |
648 |
|
649 |
@menu |
650 |
* test-i386:: |
651 |
* linux-test:: |
652 |
* qruncom.c:: |
653 |
@end menu |
654 |
|
655 |
@node test-i386 |
656 |
@section @file{test-i386} |
657 |
|
658 |
This program executes most of the 16 bit and 32 bit x86 instructions and |
659 |
generates a text output. It can be compared with the output obtained with |
660 |
a real CPU or another emulator. The target @code{make test} runs this |
661 |
program and a @code{diff} on the generated output. |
662 |
|
663 |
The Linux system call @code{modify_ldt()} is used to create x86 selectors |
664 |
to test some 16 bit addressing and 32 bit with segmentation cases. |
665 |
|
666 |
The Linux system call @code{vm86()} is used to test vm86 emulation. |
667 |
|
668 |
Various exceptions are raised to test most of the x86 user space |
669 |
exception reporting. |
670 |
|
671 |
@node linux-test |
672 |
@section @file{linux-test} |
673 |
|
674 |
This program tests various Linux system calls. It is used to verify |
675 |
that the system call parameters are correctly converted between target |
676 |
and host CPUs. |
677 |
|
678 |
@node qruncom.c |
679 |
@section @file{qruncom.c} |
680 |
|
681 |
Example of usage of @code{libqemu} to emulate a user mode i386 CPU. |
682 |
|
683 |
@node Index |
684 |
@chapter Index |
685 |
@printindex cp |
686 |
|
687 |
@bye |