File:  [Qemu by Fabrice Bellard] / qemu / qemu-tech.texi
Revision (vendor branch): download - view: text, annotated - select for diffs
Tue Apr 24 18:34:14 2018 UTC (3 years, 5 months ago) by root
Branches: qemu, MAIN
CVS tags: qemu1000, qemu0151, qemu0150, qemu0141, qemu0140, HEAD
qemu 0.14.0

    1: \input texinfo @c -*- texinfo -*-
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    3: @setfilename
    5: @documentlanguage en
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    8: @settitle QEMU Internals
    9: @exampleindent 0
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   11: @c %**end of header
   13: @ifinfo
   14: @direntry
   15: * QEMU Internals: (qemu-tech).   The QEMU Emulator Internals.
   16: @end direntry
   17: @end ifinfo
   19: @iftex
   20: @titlepage
   21: @sp 7
   22: @center @titlefont{QEMU Internals}
   23: @sp 3
   24: @end titlepage
   25: @end iftex
   27: @ifnottex
   28: @node Top
   29: @top
   31: @menu
   32: * Introduction::
   33: * QEMU Internals::
   34: * Regression Tests::
   35: * Index::
   36: @end menu
   37: @end ifnottex
   39: @contents
   41: @node Introduction
   42: @chapter Introduction
   44: @menu
   45: * intro_features::        Features
   46: * intro_x86_emulation::   x86 and x86-64 emulation
   47: * intro_arm_emulation::   ARM emulation
   48: * intro_mips_emulation::  MIPS emulation
   49: * intro_ppc_emulation::   PowerPC emulation
   50: * intro_sparc_emulation:: Sparc32 and Sparc64 emulation
   51: * intro_other_emulation:: Other CPU emulation
   52: @end menu
   54: @node intro_features
   55: @section Features
   57: QEMU is a FAST! processor emulator using a portable dynamic
   58: translator.
   60: QEMU has two operating modes:
   62: @itemize @minus
   64: @item
   65: Full system emulation. In this mode (full platform virtualization),
   66: QEMU emulates a full system (usually a PC), including a processor and
   67: various peripherals. It can be used to launch several different
   68: Operating Systems at once without rebooting the host machine or to
   69: debug system code.
   71: @item
   72: User mode emulation. In this mode (application level virtualization),
   73: QEMU can launch processes compiled for one CPU on another CPU, however
   74: the Operating Systems must match. This can be used for example to ease
   75: cross-compilation and cross-debugging.
   76: @end itemize
   78: As QEMU requires no host kernel driver to run, it is very safe and
   79: easy to use.
   81: QEMU generic features:
   83: @itemize
   85: @item User space only or full system emulation.
   87: @item Using dynamic translation to native code for reasonable speed.
   89: @item
   90: Working on x86, x86_64 and PowerPC32/64 hosts. Being tested on ARM,
   91: HPPA, Sparc32 and Sparc64. Previous versions had some support for
   92: Alpha and S390 hosts, but TCG (see below) doesn't support those yet.
   94: @item Self-modifying code support.
   96: @item Precise exceptions support.
   98: @item The virtual CPU is a library (@code{libqemu}) which can be used
   99: in other projects (look at @file{qemu/tests/qruncom.c} to have an
  100: example of user mode @code{libqemu} usage).
  102: @item
  103: Floating point library supporting both full software emulation and
  104: native host FPU instructions.
  106: @end itemize
  108: QEMU user mode emulation features:
  109: @itemize
  110: @item Generic Linux system call converter, including most ioctls.
  112: @item clone() emulation using native CPU clone() to use Linux scheduler for threads.
  114: @item Accurate signal handling by remapping host signals to target signals.
  115: @end itemize
  117: Linux user emulator (Linux host only) can be used to launch the Wine
  118: Windows API emulator (@url{}). A Darwin user
  119: emulator (Darwin hosts only) exists and a BSD user emulator for BSD
  120: hosts is under development. It would also be possible to develop a
  121: similar user emulator for Solaris.
  123: QEMU full system emulation features:
  124: @itemize
  125: @item
  126: QEMU uses a full software MMU for maximum portability.
  128: @item
  129: QEMU can optionally use an in-kernel accelerator, like kvm. The accelerators 
  130: execute some of the guest code natively, while
  131: continuing to emulate the rest of the machine.
  133: @item
  134: Various hardware devices can be emulated and in some cases, host
  135: devices (e.g. serial and parallel ports, USB, drives) can be used
  136: transparently by the guest Operating System. Host device passthrough
  137: can be used for talking to external physical peripherals (e.g. a
  138: webcam, modem or tape drive).
  140: @item
  141: Symmetric multiprocessing (SMP) even on a host with a single CPU. On a
  142: SMP host system, QEMU can use only one CPU fully due to difficulty in
  143: implementing atomic memory accesses efficiently.
  145: @end itemize
  147: @node intro_x86_emulation
  148: @section x86 and x86-64 emulation
  150: QEMU x86 target features:
  152: @itemize
  154: @item The virtual x86 CPU supports 16 bit and 32 bit addressing with segmentation.
  155: LDT/GDT and IDT are emulated. VM86 mode is also supported to run
  156: DOSEMU. There is some support for MMX/3DNow!, SSE, SSE2, SSE3, SSSE3,
  157: and SSE4 as well as x86-64 SVM.
  159: @item Support of host page sizes bigger than 4KB in user mode emulation.
  161: @item QEMU can emulate itself on x86.
  163: @item An extensive Linux x86 CPU test program is included @file{tests/test-i386}.
  164: It can be used to test other x86 virtual CPUs.
  166: @end itemize
  168: Current QEMU limitations:
  170: @itemize
  172: @item Limited x86-64 support.
  174: @item IPC syscalls are missing.
  176: @item The x86 segment limits and access rights are not tested at every
  177: memory access (yet). Hopefully, very few OSes seem to rely on that for
  178: normal use.
  180: @end itemize
  182: @node intro_arm_emulation
  183: @section ARM emulation
  185: @itemize
  187: @item Full ARM 7 user emulation.
  189: @item NWFPE FPU support included in user Linux emulation.
  191: @item Can run most ARM Linux binaries.
  193: @end itemize
  195: @node intro_mips_emulation
  196: @section MIPS emulation
  198: @itemize
  200: @item The system emulation allows full MIPS32/MIPS64 Release 2 emulation,
  201: including privileged instructions, FPU and MMU, in both little and big
  202: endian modes.
  204: @item The Linux userland emulation can run many 32 bit MIPS Linux binaries.
  206: @end itemize
  208: Current QEMU limitations:
  210: @itemize
  212: @item Self-modifying code is not always handled correctly.
  214: @item 64 bit userland emulation is not implemented.
  216: @item The system emulation is not complete enough to run real firmware.
  218: @item The watchpoint debug facility is not implemented.
  220: @end itemize
  222: @node intro_ppc_emulation
  223: @section PowerPC emulation
  225: @itemize
  227: @item Full PowerPC 32 bit emulation, including privileged instructions,
  228: FPU and MMU.
  230: @item Can run most PowerPC Linux binaries.
  232: @end itemize
  234: @node intro_sparc_emulation
  235: @section Sparc32 and Sparc64 emulation
  237: @itemize
  239: @item Full SPARC V8 emulation, including privileged
  240: instructions, FPU and MMU. SPARC V9 emulation includes most privileged
  241: and VIS instructions, FPU and I/D MMU. Alignment is fully enforced.
  243: @item Can run most 32-bit SPARC Linux binaries, SPARC32PLUS Linux binaries and
  244: some 64-bit SPARC Linux binaries.
  246: @end itemize
  248: Current QEMU limitations:
  250: @itemize
  252: @item IPC syscalls are missing.
  254: @item Floating point exception support is buggy.
  256: @item Atomic instructions are not correctly implemented.
  258: @item There are still some problems with Sparc64 emulators.
  260: @end itemize
  262: @node intro_other_emulation
  263: @section Other CPU emulation
  265: In addition to the above, QEMU supports emulation of other CPUs with
  266: varying levels of success. These are:
  268: @itemize
  270: @item
  271: Alpha
  272: @item
  273: CRIS
  274: @item
  275: M68k
  276: @item
  277: SH4
  278: @end itemize
  280: @node QEMU Internals
  281: @chapter QEMU Internals
  283: @menu
  284: * QEMU compared to other emulators::
  285: * Portable dynamic translation::
  286: * Condition code optimisations::
  287: * CPU state optimisations::
  288: * Translation cache::
  289: * Direct block chaining::
  290: * Self-modifying code and translated code invalidation::
  291: * Exception support::
  292: * MMU emulation::
  293: * Device emulation::
  294: * Hardware interrupts::
  295: * User emulation specific details::
  296: * Bibliography::
  297: @end menu
  299: @node QEMU compared to other emulators
  300: @section QEMU compared to other emulators
  302: Like bochs [3], QEMU emulates an x86 CPU. But QEMU is much faster than
  303: bochs as it uses dynamic compilation. Bochs is closely tied to x86 PC
  304: emulation while QEMU can emulate several processors.
  306: Like Valgrind [2], QEMU does user space emulation and dynamic
  307: translation. Valgrind is mainly a memory debugger while QEMU has no
  308: support for it (QEMU could be used to detect out of bound memory
  309: accesses as Valgrind, but it has no support to track uninitialised data
  310: as Valgrind does). The Valgrind dynamic translator generates better code
  311: than QEMU (in particular it does register allocation) but it is closely
  312: tied to an x86 host and target and has no support for precise exceptions
  313: and system emulation.
  315: EM86 [4] is the closest project to user space QEMU (and QEMU still uses
  316: some of its code, in particular the ELF file loader). EM86 was limited
  317: to an alpha host and used a proprietary and slow interpreter (the
  318: interpreter part of the FX!32 Digital Win32 code translator [5]).
  320: TWIN [6] is a Windows API emulator like Wine. It is less accurate than
  321: Wine but includes a protected mode x86 interpreter to launch x86 Windows
  322: executables. Such an approach has greater potential because most of the
  323: Windows API is executed natively but it is far more difficult to develop
  324: because all the data structures and function parameters exchanged
  325: between the API and the x86 code must be converted.
  327: User mode Linux [7] was the only solution before QEMU to launch a
  328: Linux kernel as a process while not needing any host kernel
  329: patches. However, user mode Linux requires heavy kernel patches while
  330: QEMU accepts unpatched Linux kernels. The price to pay is that QEMU is
  331: slower.
  333: The Plex86 [8] PC virtualizer is done in the same spirit as the now
  334: obsolete qemu-fast system emulator. It requires a patched Linux kernel
  335: to work (you cannot launch the same kernel on your PC), but the
  336: patches are really small. As it is a PC virtualizer (no emulation is
  337: done except for some privileged instructions), it has the potential of
  338: being faster than QEMU. The downside is that a complicated (and
  339: potentially unsafe) host kernel patch is needed.
  341: The commercial PC Virtualizers (VMWare [9], VirtualPC [10], TwoOStwo
  342: [11]) are faster than QEMU, but they all need specific, proprietary
  343: and potentially unsafe host drivers. Moreover, they are unable to
  344: provide cycle exact simulation as an emulator can.
  346: VirtualBox [12], Xen [13] and KVM [14] are based on QEMU. QEMU-SystemC
  347: [15] uses QEMU to simulate a system where some hardware devices are
  348: developed in SystemC.
  350: @node Portable dynamic translation
  351: @section Portable dynamic translation
  353: QEMU is a dynamic translator. When it first encounters a piece of code,
  354: it converts it to the host instruction set. Usually dynamic translators
  355: are very complicated and highly CPU dependent. QEMU uses some tricks
  356: which make it relatively easily portable and simple while achieving good
  357: performances.
  359: After the release of version 0.9.1, QEMU switched to a new method of
  360: generating code, Tiny Code Generator or TCG. TCG relaxes the
  361: dependency on the exact version of the compiler used. The basic idea
  362: is to split every target instruction into a couple of RISC-like TCG
  363: ops (see @code{target-i386/translate.c}). Some optimizations can be
  364: performed at this stage, including liveness analysis and trivial
  365: constant expression evaluation. TCG ops are then implemented in the
  366: host CPU back end, also known as TCG target (see
  367: @code{tcg/i386/tcg-target.c}). For more information, please take a
  368: look at @code{tcg/README}.
  370: @node Condition code optimisations
  371: @section Condition code optimisations
  373: Lazy evaluation of CPU condition codes (@code{EFLAGS} register on x86)
  374: is important for CPUs where every instruction sets the condition
  375: codes. It tends to be less important on conventional RISC systems
  376: where condition codes are only updated when explicitly requested. On
  377: Sparc64, costly update of both 32 and 64 bit condition codes can be
  378: avoided with lazy evaluation.
  380: Instead of computing the condition codes after each x86 instruction,
  381: QEMU just stores one operand (called @code{CC_SRC}), the result
  382: (called @code{CC_DST}) and the type of operation (called
  383: @code{CC_OP}). When the condition codes are needed, the condition
  384: codes can be calculated using this information. In addition, an
  385: optimized calculation can be performed for some instruction types like
  386: conditional branches.
  388: @code{CC_OP} is almost never explicitly set in the generated code
  389: because it is known at translation time.
  391: The lazy condition code evaluation is used on x86, m68k, cris and
  392: Sparc. ARM uses a simplified variant for the N and Z flags.
  394: @node CPU state optimisations
  395: @section CPU state optimisations
  397: The target CPUs have many internal states which change the way it
  398: evaluates instructions. In order to achieve a good speed, the
  399: translation phase considers that some state information of the virtual
  400: CPU cannot change in it. The state is recorded in the Translation
  401: Block (TB). If the state changes (e.g. privilege level), a new TB will
  402: be generated and the previous TB won't be used anymore until the state
  403: matches the state recorded in the previous TB. For example, if the SS,
  404: DS and ES segments have a zero base, then the translator does not even
  405: generate an addition for the segment base.
  407: [The FPU stack pointer register is not handled that way yet].
  409: @node Translation cache
  410: @section Translation cache
  412: A 16 MByte cache holds the most recently used translations. For
  413: simplicity, it is completely flushed when it is full. A translation unit
  414: contains just a single basic block (a block of x86 instructions
  415: terminated by a jump or by a virtual CPU state change which the
  416: translator cannot deduce statically).
  418: @node Direct block chaining
  419: @section Direct block chaining
  421: After each translated basic block is executed, QEMU uses the simulated
  422: Program Counter (PC) and other cpu state informations (such as the CS
  423: segment base value) to find the next basic block.
  425: In order to accelerate the most common cases where the new simulated PC
  426: is known, QEMU can patch a basic block so that it jumps directly to the
  427: next one.
  429: The most portable code uses an indirect jump. An indirect jump makes
  430: it easier to make the jump target modification atomic. On some host
  431: architectures (such as x86 or PowerPC), the @code{JUMP} opcode is
  432: directly patched so that the block chaining has no overhead.
  434: @node Self-modifying code and translated code invalidation
  435: @section Self-modifying code and translated code invalidation
  437: Self-modifying code is a special challenge in x86 emulation because no
  438: instruction cache invalidation is signaled by the application when code
  439: is modified.
  441: When translated code is generated for a basic block, the corresponding
  442: host page is write protected if it is not already read-only. Then, if
  443: a write access is done to the page, Linux raises a SEGV signal. QEMU
  444: then invalidates all the translated code in the page and enables write
  445: accesses to the page.
  447: Correct translated code invalidation is done efficiently by maintaining
  448: a linked list of every translated block contained in a given page. Other
  449: linked lists are also maintained to undo direct block chaining.
  451: On RISC targets, correctly written software uses memory barriers and
  452: cache flushes, so some of the protection above would not be
  453: necessary. However, QEMU still requires that the generated code always
  454: matches the target instructions in memory in order to handle
  455: exceptions correctly.
  457: @node Exception support
  458: @section Exception support
  460: longjmp() is used when an exception such as division by zero is
  461: encountered.
  463: The host SIGSEGV and SIGBUS signal handlers are used to get invalid
  464: memory accesses. The simulated program counter is found by
  465: retranslating the corresponding basic block and by looking where the
  466: host program counter was at the exception point.
  468: The virtual CPU cannot retrieve the exact @code{EFLAGS} register because
  469: in some cases it is not computed because of condition code
  470: optimisations. It is not a big concern because the emulated code can
  471: still be restarted in any cases.
  473: @node MMU emulation
  474: @section MMU emulation
  476: For system emulation QEMU supports a soft MMU. In that mode, the MMU
  477: virtual to physical address translation is done at every memory
  478: access. QEMU uses an address translation cache to speed up the
  479: translation.
  481: In order to avoid flushing the translated code each time the MMU
  482: mappings change, QEMU uses a physically indexed translation cache. It
  483: means that each basic block is indexed with its physical address.
  485: When MMU mappings change, only the chaining of the basic blocks is
  486: reset (i.e. a basic block can no longer jump directly to another one).
  488: @node Device emulation
  489: @section Device emulation
  491: Systems emulated by QEMU are organized by boards. At initialization
  492: phase, each board instantiates a number of CPUs, devices, RAM and
  493: ROM. Each device in turn can assign I/O ports or memory areas (for
  494: MMIO) to its handlers. When the emulation starts, an access to the
  495: ports or MMIO memory areas assigned to the device causes the
  496: corresponding handler to be called.
  498: RAM and ROM are handled more optimally, only the offset to the host
  499: memory needs to be added to the guest address.
  501: The video RAM of VGA and other display cards is special: it can be
  502: read or written directly like RAM, but write accesses cause the memory
  503: to be marked with VGA_DIRTY flag as well.
  505: QEMU supports some device classes like serial and parallel ports, USB,
  506: drives and network devices, by providing APIs for easier connection to
  507: the generic, higher level implementations. The API hides the
  508: implementation details from the devices, like native device use or
  509: advanced block device formats like QCOW.
  511: Usually the devices implement a reset method and register support for
  512: saving and loading of the device state. The devices can also use
  513: timers, especially together with the use of bottom halves (BHs).
  515: @node Hardware interrupts
  516: @section Hardware interrupts
  518: In order to be faster, QEMU does not check at every basic block if an
  519: hardware interrupt is pending. Instead, the user must asynchronously
  520: call a specific function to tell that an interrupt is pending. This
  521: function resets the chaining of the currently executing basic
  522: block. It ensures that the execution will return soon in the main loop
  523: of the CPU emulator. Then the main loop can test if the interrupt is
  524: pending and handle it.
  526: @node User emulation specific details
  527: @section User emulation specific details
  529: @subsection Linux system call translation
  531: QEMU includes a generic system call translator for Linux. It means that
  532: the parameters of the system calls can be converted to fix the
  533: endianness and 32/64 bit issues. The IOCTLs are converted with a generic
  534: type description system (see @file{ioctls.h} and @file{thunk.c}).
  536: QEMU supports host CPUs which have pages bigger than 4KB. It records all
  537: the mappings the process does and try to emulated the @code{mmap()}
  538: system calls in cases where the host @code{mmap()} call would fail
  539: because of bad page alignment.
  541: @subsection Linux signals
  543: Normal and real-time signals are queued along with their information
  544: (@code{siginfo_t}) as it is done in the Linux kernel. Then an interrupt
  545: request is done to the virtual CPU. When it is interrupted, one queued
  546: signal is handled by generating a stack frame in the virtual CPU as the
  547: Linux kernel does. The @code{sigreturn()} system call is emulated to return
  548: from the virtual signal handler.
  550: Some signals (such as SIGALRM) directly come from the host. Other
  551: signals are synthesized from the virtual CPU exceptions such as SIGFPE
  552: when a division by zero is done (see @code{main.c:cpu_loop()}).
  554: The blocked signal mask is still handled by the host Linux kernel so
  555: that most signal system calls can be redirected directly to the host
  556: Linux kernel. Only the @code{sigaction()} and @code{sigreturn()} system
  557: calls need to be fully emulated (see @file{signal.c}).
  559: @subsection clone() system call and threads
  561: The Linux clone() system call is usually used to create a thread. QEMU
  562: uses the host clone() system call so that real host threads are created
  563: for each emulated thread. One virtual CPU instance is created for each
  564: thread.
  566: The virtual x86 CPU atomic operations are emulated with a global lock so
  567: that their semantic is preserved.
  569: Note that currently there are still some locking issues in QEMU. In
  570: particular, the translated cache flush is not protected yet against
  571: reentrancy.
  573: @subsection Self-virtualization
  575: QEMU was conceived so that ultimately it can emulate itself. Although
  576: it is not very useful, it is an important test to show the power of the
  577: emulator.
  579: Achieving self-virtualization is not easy because there may be address
  580: space conflicts. QEMU user emulators solve this problem by being an
  581: executable ELF shared object as the ELF interpreter. That
  582: way, it can be relocated at load time.
  584: @node Bibliography
  585: @section Bibliography
  587: @table @asis
  589: @item [1]
  590: @url{}, Optimizing
  591: direct threaded code by selective inlining (1998) by Ian Piumarta, Fabio
  592: Riccardi.
  594: @item [2]
  595: @url{}, Valgrind, an open-source
  596: memory debugger for x86-GNU/Linux, by Julian Seward.
  598: @item [3]
  599: @url{}, the Bochs IA-32 Emulator Project,
  600: by Kevin Lawton et al.
  602: @item [4]
  603: @url{}, the EM86
  604: x86 emulator on Alpha-Linux.
  606: @item [5]
  607: @url{},
  608: DIGITAL FX!32: Running 32-Bit x86 Applications on Alpha NT, by Anton
  609: Chernoff and Ray Hookway.
  611: @item [6]
  612: @url{}, Windows API library emulation from
  613: Willows Software.
  615: @item [7]
  616: @url{},
  617: The User-mode Linux Kernel.
  619: @item [8]
  620: @url{},
  621: The new Plex86 project.
  623: @item [9]
  624: @url{},
  625: The VMWare PC virtualizer.
  627: @item [10]
  628: @url{},
  629: The VirtualPC PC virtualizer.
  631: @item [11]
  632: @url{},
  633: The TwoOStwo PC virtualizer.
  635: @item [12]
  636: @url{},
  637: The VirtualBox PC virtualizer.
  639: @item [13]
  640: @url{},
  641: The Xen hypervisor.
  643: @item [14]
  644: @url{},
  645: Kernel Based Virtual Machine (KVM).
  647: @item [15]
  648: @url{},
  649: QEMU-SystemC, a hardware co-simulator.
  651: @end table
  653: @node Regression Tests
  654: @chapter Regression Tests
  656: In the directory @file{tests/}, various interesting testing programs
  657: are available. They are used for regression testing.
  659: @menu
  660: * test-i386::
  661: * linux-test::
  662: * qruncom.c::
  663: @end menu
  665: @node test-i386
  666: @section @file{test-i386}
  668: This program executes most of the 16 bit and 32 bit x86 instructions and
  669: generates a text output. It can be compared with the output obtained with
  670: a real CPU or another emulator. The target @code{make test} runs this
  671: program and a @code{diff} on the generated output.
  673: The Linux system call @code{modify_ldt()} is used to create x86 selectors
  674: to test some 16 bit addressing and 32 bit with segmentation cases.
  676: The Linux system call @code{vm86()} is used to test vm86 emulation.
  678: Various exceptions are raised to test most of the x86 user space
  679: exception reporting.
  681: @node linux-test
  682: @section @file{linux-test}
  684: This program tests various Linux system calls. It is used to verify
  685: that the system call parameters are correctly converted between target
  686: and host CPUs.
  688: @node qruncom.c
  689: @section @file{qruncom.c}
  691: Example of usage of @code{libqemu} to emulate a user mode i386 CPU.
  693: @node Index
  694: @chapter Index
  695: @printindex cp
  697: @bye