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\input texinfo @c -*- texinfo -*- |
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@settitle QEMU x86 Emulator Reference Documentation |
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@titlepage |
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@sp 7 |
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@center @titlefont{QEMU x86 Emulator Reference Documentation} |
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@sp 3 |
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@end titlepage |
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|
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@chapter Introduction |
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|
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QEMU is an x86 processor emulator. Its purpose is to run x86 Linux |
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processes on non-x86 Linux architectures such as PowerPC or ARM. By |
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using dynamic translation it achieves a reasonnable speed while being |
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easy to port on new host CPUs. An obviously interesting x86 only process |
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is 'wine' (Windows emulation). |
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|
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QEMU features: |
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|
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@itemize |
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|
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@item User space only x86 emulator. |
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|
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@item Currently ported on i386 and PowerPC. |
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|
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@item Using dynamic translation for reasonnable speed. |
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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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User space LDT and GDT are emulated. |
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|
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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 virtual x86 signals. |
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|
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@item The virtual x86 CPU is a library (@code{libqemu}) which can be used |
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in other projects. |
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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 Not all x86 exceptions are precise (yet). [Very few programs need that]. |
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|
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@item Not self virtualizable (yet). [You cannot launch qemu with qemu on the same CPU]. |
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|
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@item No support for self modifying code (yet). [Very few programs need that, a notable exception is QEMU itself !]. |
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|
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@item No VM86 mode (yet), althought the virtual |
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CPU has support for most of it. [VM86 support is useful to launch old 16 |
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bit DOS programs with dosemu or wine]. |
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|
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@item No SSE/MMX support (yet). |
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|
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@item No x86-64 support. |
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|
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@item Some Linux 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 (and will never be to have good performances). |
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|
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@item On non x86 host CPUs, @code{double}s are used instead of the non standard |
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10 byte @code{long double}s of x86 for floating point emulation to get |
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maximum performances. |
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|
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@end itemize |
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|
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@chapter Invocation |
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|
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In order to launch a Linux process, QEMU needs the process executable |
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itself and all the target (x86) dynamic libraries used by it. Currently, |
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QEMU is not distributed with the necessary packages so that you can test |
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it easily on non x86 CPUs. |
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|
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However, the statically x86 binary 'tests/hello' can be used to do a |
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first test: |
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|
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@example |
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qemu tests/hello |
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@end example |
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|
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@code{Hello world} should be printed on the terminal. |
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|
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If you are testing it on a x86 CPU, then you can test it on any process: |
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|
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@example |
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qemu /bin/ls -l |
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@end example |
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|
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@chapter QEMU Internals |
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|
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@section QEMU compared to other emulators |
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|
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Unlike bochs [3], QEMU emulates only a user space x86 CPU. It means that |
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you cannot launch an operating system with it. The benefit is that it is |
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simpler and faster due to the fact that some of the low level CPU state |
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can be ignored (in particular, no virtual memory needs to be emulated). |
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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 accesses |
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as Valgrind, but it has no support to track uninitialised data as |
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Valgrind does). Valgrind dynamic translator generates better code than |
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QEMU (in particular it does register allocation) but it is closely tied |
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to an x86 host. |
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|
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EM86 [4] is the closest project to QEMU (and QEMU still uses some of its |
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code, in particular the ELF file loader). EM86 was limited to an alpha |
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host and used a proprietary and slow interpreter (the interpreter part |
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of the FX!32 Digital Win32 code translator [5]). |
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|
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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 dependant. 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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The basic idea is to split every x86 instruction into fewer simpler |
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instructions. Each simple instruction is implemented by a piece of C |
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code (see @file{op-i386.c}). Then a compile time tool (@file{dyngen}) |
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takes the corresponding object file (@file{op-i386.o}) to generate a |
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dynamic code generator which concatenates the simple instructions to |
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build a function (see @file{op-i386.h:dyngen_code()}). |
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|
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In essence, the process is similar to [1], but more work is done at |
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compile time. |
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|
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A key idea to get optimal performances is that constant parameters can |
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be passed to the simple operations. For that purpose, dummy ELF |
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relocations are generated with gcc for each constant parameter. Then, |
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the tool (@file{dyngen}) can locate the relocations and generate the |
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appriopriate C code to resolve them when building the dynamic code. |
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|
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That way, QEMU is no more difficult to port than a dynamic linker. |
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|
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To go even faster, GCC static register variables are used to keep the |
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state of the virtual CPU. |
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|
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@section Register allocation |
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|
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Since QEMU uses fixed simple instructions, no efficient register |
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allocation can be done. However, because RISC CPUs have a lot of |
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register, most of the virtual CPU state can be put in registers without |
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doing complicated register allocation. |
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|
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@section Condition code optimisations |
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|
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Good CPU condition codes emulation (@code{EFLAGS} register on x86) is a |
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critical point to get good performances. QEMU uses lazy condition code |
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evaluation: instead of computing the condition codes after each x86 |
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instruction, it store justs one operand (called @code{CC_CRC}), the |
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result (called @code{CC_DST}) and the type of operation (called |
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@code{CC_OP}). |
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|
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@code{CC_OP} is almost never explicitely set in the generated code |
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because it is known at translation time. |
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|
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In order to increase performances, a backward pass is performed on the |
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generated simple instructions (see |
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@code{translate-i386.c:optimize_flags()}). When it can be proved that |
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the condition codes are not needed by the next instructions, no |
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condition codes are computed at all. |
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|
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@section Translation CPU state optimisations |
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|
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The x86 CPU has many internal states which change the way it evaluates |
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instructions. In order to achieve a good speed, the translation phase |
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considers that some state information of the virtual x86 CPU cannot |
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change in it. For example, if the SS, DS and ES segments have a zero |
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base, then the translator does not even generate an addition for the |
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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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@section Translation cache |
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|
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A 2MByte 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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[Currently, the translated code is not patched if it jumps to another |
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translated code]. |
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|
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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. The host SIGSEGV and SIGBUS signal handlers are used to get |
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invalid memory accesses. |
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|
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[Currently, the virtual CPU cannot retrieve the exact CPU state in some |
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exceptions, although it could except for the @code{EFLAGS} register]. |
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@section Linux system call translation |
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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 |
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type description system (see @file{ioctls.h} and @file{thunk.c}). |
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|
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@section 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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|
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The blocked signal mask is still handled by the host Linux kernel so |
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that most signal system calls can be redirected directly to the host |
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Linux kernel. Only the @code{sigaction()} and @code{sigreturn()} system |
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calls need to be fully emulated (see @file{signal.c}). |
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|
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@section clone() system call and threads |
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|
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The Linux clone() system call is usually used to create a thread. QEMU |
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uses the host clone() system call so that real host threads are created |
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for each emulated thread. One virtual CPU instance is created for each |
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thread. |
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|
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The virtual x86 CPU atomic operations are emulated with a global lock so |
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that their semantic is preserved. |
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|
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@section Bibliography |
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|
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@table @asis |
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|
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@item [1] |
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@url{http://citeseer.nj.nec.com/piumarta98optimizing.html}, Optimizing |
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direct threaded code by selective inlining (1998) by Ian Piumarta, Fabio |
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Riccardi. |
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|
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@item [2] |
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@url{http://developer.kde.org/~sewardj/}, Valgrind, an open-source |
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memory debugger for x86-GNU/Linux, by Julian Seward. |
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|
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@item [3] |
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@url{http://bochs.sourceforge.net/}, the Bochs IA-32 Emulator Project, |
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by Kevin Lawton et al. |
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|
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@item [4] |
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@url{http://www.cs.rose-hulman.edu/~donaldlf/em86/index.html}, the EM86 |
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x86 emulator on Alpha-Linux. |
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|
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@item [5] |
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@url{http://www.usenix.org/publications/library/proceedings/usenix-nt97/full_papers/chernoff/chernoff.pdf}, |
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DIGITAL FX!32: Running 32-Bit x86 Applications on Alpha NT, by Anton |
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Chernoff and Ray Hookway. |
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|
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@end table |
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|
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@chapter Regression Tests |
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|
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In the directory @file{tests/}, various interesting x86 testing programs |
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are available. There are used for regression testing. |
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@section @file{hello} |
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Very simple statically linked x86 program, just to test QEMU during a |
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port to a new host CPU. |
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@section @file{test-i386} |
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|
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This program executes most of the 16 bit and 32 bit x86 instructions and |
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generates a text output. It can be compared with the output obtained with |
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a real CPU or another emulator. The target @code{make test} runs this |
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program and a @code{diff} on the generated output. |
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|
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The Linux system call @code{modify_ldt()} is used to create x86 selectors |
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to test some 16 bit addressing and 32 bit with segmentation cases. |
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@section @file{testsig} |
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|
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This program tests various signal cases, including SIGFPE, SIGSEGV and |
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SIGILL. |
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@section @file{testclone} |
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Tests the @code{clone()} system call (basic test). |
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@section @file{testthread} |
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|
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Tests the glibc threads (more complicated than @code{clone()} because signals |
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are also used). |
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@section @file{sha1} |
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|
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It is a simple benchmark. Care must be taken to interpret the results |
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because it mostly tests the ability of the virtual CPU to optimize the |
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@code{rol} x86 instruction and the condition code computations. |
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|