qemu/qemu-tech.texi
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   3@setfilename qemu-tech.info
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   8@settitle QEMU Internals
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  13@ifinfo
  14@direntry
  15* QEMU Internals: (qemu-tech).   The QEMU Emulator Internals.
  16@end direntry
  17@end ifinfo
  18
  19@iftex
  20@titlepage
  21@sp 7
  22@center @titlefont{QEMU Internals}
  23@sp 3
  24@end titlepage
  25@end iftex
  26
  27@ifnottex
  28@node Top
  29@top
  30
  31@menu
  32* Introduction::
  33* QEMU Internals::
  34* Regression Tests::
  35* Index::
  36@end menu
  37@end ifnottex
  38
  39@contents
  40
  41@node Introduction
  42@chapter Introduction
  43
  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_xtensa_emulation:: Xtensa emulation
  52* intro_other_emulation::  Other CPU emulation
  53@end menu
  54
  55@node intro_features
  56@section Features
  57
  58QEMU is a FAST! processor emulator using a portable dynamic
  59translator.
  60
  61QEMU has two operating modes:
  62
  63@itemize @minus
  64
  65@item
  66Full system emulation. In this mode (full platform virtualization),
  67QEMU emulates a full system (usually a PC), including a processor and
  68various peripherals. It can be used to launch several different
  69Operating Systems at once without rebooting the host machine or to
  70debug system code.
  71
  72@item
  73User mode emulation. In this mode (application level virtualization),
  74QEMU can launch processes compiled for one CPU on another CPU, however
  75the Operating Systems must match. This can be used for example to ease
  76cross-compilation and cross-debugging.
  77@end itemize
  78
  79As QEMU requires no host kernel driver to run, it is very safe and
  80easy to use.
  81
  82QEMU generic features:
  83
  84@itemize
  85
  86@item User space only or full system emulation.
  87
  88@item Using dynamic translation to native code for reasonable speed.
  89
  90@item
  91Working on x86, x86_64 and PowerPC32/64 hosts. Being tested on ARM,
  92HPPA, Sparc32 and Sparc64. Previous versions had some support for
  93Alpha and S390 hosts, but TCG (see below) doesn't support those yet.
  94
  95@item Self-modifying code support.
  96
  97@item Precise exceptions support.
  98
  99@item
 100Floating point library supporting both full software emulation and
 101native host FPU instructions.
 102
 103@end itemize
 104
 105QEMU user mode emulation features:
 106@itemize
 107@item Generic Linux system call converter, including most ioctls.
 108
 109@item clone() emulation using native CPU clone() to use Linux scheduler for threads.
 110
 111@item Accurate signal handling by remapping host signals to target signals.
 112@end itemize
 113
 114Linux user emulator (Linux host only) can be used to launch the Wine
 115Windows API emulator (@url{http://www.winehq.org}). A BSD user emulator for BSD
 116hosts is under development. It would also be possible to develop a
 117similar user emulator for Solaris.
 118
 119QEMU full system emulation features:
 120@itemize
 121@item
 122QEMU uses a full software MMU for maximum portability.
 123
 124@item
 125QEMU can optionally use an in-kernel accelerator, like kvm. The accelerators 
 126execute some of the guest code natively, while
 127continuing to emulate the rest of the machine.
 128
 129@item
 130Various hardware devices can be emulated and in some cases, host
 131devices (e.g. serial and parallel ports, USB, drives) can be used
 132transparently by the guest Operating System. Host device passthrough
 133can be used for talking to external physical peripherals (e.g. a
 134webcam, modem or tape drive).
 135
 136@item
 137Symmetric multiprocessing (SMP) even on a host with a single CPU. On a
 138SMP host system, QEMU can use only one CPU fully due to difficulty in
 139implementing atomic memory accesses efficiently.
 140
 141@end itemize
 142
 143@node intro_x86_emulation
 144@section x86 and x86-64 emulation
 145
 146QEMU x86 target features:
 147
 148@itemize
 149
 150@item The virtual x86 CPU supports 16 bit and 32 bit addressing with segmentation.
 151LDT/GDT and IDT are emulated. VM86 mode is also supported to run
 152DOSEMU. There is some support for MMX/3DNow!, SSE, SSE2, SSE3, SSSE3,
 153and SSE4 as well as x86-64 SVM.
 154
 155@item Support of host page sizes bigger than 4KB in user mode emulation.
 156
 157@item QEMU can emulate itself on x86.
 158
 159@item An extensive Linux x86 CPU test program is included @file{tests/test-i386}.
 160It can be used to test other x86 virtual CPUs.
 161
 162@end itemize
 163
 164Current QEMU limitations:
 165
 166@itemize
 167
 168@item Limited x86-64 support.
 169
 170@item IPC syscalls are missing.
 171
 172@item The x86 segment limits and access rights are not tested at every
 173memory access (yet). Hopefully, very few OSes seem to rely on that for
 174normal use.
 175
 176@end itemize
 177
 178@node intro_arm_emulation
 179@section ARM emulation
 180
 181@itemize
 182
 183@item Full ARM 7 user emulation.
 184
 185@item NWFPE FPU support included in user Linux emulation.
 186
 187@item Can run most ARM Linux binaries.
 188
 189@end itemize
 190
 191@node intro_mips_emulation
 192@section MIPS emulation
 193
 194@itemize
 195
 196@item The system emulation allows full MIPS32/MIPS64 Release 2 emulation,
 197including privileged instructions, FPU and MMU, in both little and big
 198endian modes.
 199
 200@item The Linux userland emulation can run many 32 bit MIPS Linux binaries.
 201
 202@end itemize
 203
 204Current QEMU limitations:
 205
 206@itemize
 207
 208@item Self-modifying code is not always handled correctly.
 209
 210@item 64 bit userland emulation is not implemented.
 211
 212@item The system emulation is not complete enough to run real firmware.
 213
 214@item The watchpoint debug facility is not implemented.
 215
 216@end itemize
 217
 218@node intro_ppc_emulation
 219@section PowerPC emulation
 220
 221@itemize
 222
 223@item Full PowerPC 32 bit emulation, including privileged instructions,
 224FPU and MMU.
 225
 226@item Can run most PowerPC Linux binaries.
 227
 228@end itemize
 229
 230@node intro_sparc_emulation
 231@section Sparc32 and Sparc64 emulation
 232
 233@itemize
 234
 235@item Full SPARC V8 emulation, including privileged
 236instructions, FPU and MMU. SPARC V9 emulation includes most privileged
 237and VIS instructions, FPU and I/D MMU. Alignment is fully enforced.
 238
 239@item Can run most 32-bit SPARC Linux binaries, SPARC32PLUS Linux binaries and
 240some 64-bit SPARC Linux binaries.
 241
 242@end itemize
 243
 244Current QEMU limitations:
 245
 246@itemize
 247
 248@item IPC syscalls are missing.
 249
 250@item Floating point exception support is buggy.
 251
 252@item Atomic instructions are not correctly implemented.
 253
 254@item There are still some problems with Sparc64 emulators.
 255
 256@end itemize
 257
 258@node intro_xtensa_emulation
 259@section Xtensa emulation
 260
 261@itemize
 262
 263@item Core Xtensa ISA emulation, including most options: code density,
 264loop, extended L32R, 16- and 32-bit multiplication, 32-bit division,
 265MAC16, miscellaneous operations, boolean, FP coprocessor, coprocessor
 266context, debug, multiprocessor synchronization,
 267conditional store, exceptions, relocatable vectors, unaligned exception,
 268interrupts (including high priority and timer), hardware alignment,
 269region protection, region translation, MMU, windowed registers, thread
 270pointer, processor ID.
 271
 272@item Not implemented options: data/instruction cache (including cache
 273prefetch and locking), XLMI, processor interface. Also options not
 274covered by the core ISA (e.g. FLIX, wide branches) are not implemented.
 275
 276@item Can run most Xtensa Linux binaries.
 277
 278@item New core configuration that requires no additional instructions
 279may be created from overlay with minimal amount of hand-written code.
 280
 281@end itemize
 282
 283@node intro_other_emulation
 284@section Other CPU emulation
 285
 286In addition to the above, QEMU supports emulation of other CPUs with
 287varying levels of success. These are:
 288
 289@itemize
 290
 291@item
 292Alpha
 293@item
 294CRIS
 295@item
 296M68k
 297@item
 298SH4
 299@end itemize
 300
 301@node QEMU Internals
 302@chapter QEMU Internals
 303
 304@menu
 305* QEMU compared to other emulators::
 306* Portable dynamic translation::
 307* Condition code optimisations::
 308* CPU state optimisations::
 309* Translation cache::
 310* Direct block chaining::
 311* Self-modifying code and translated code invalidation::
 312* Exception support::
 313* MMU emulation::
 314* Device emulation::
 315* Hardware interrupts::
 316* User emulation specific details::
 317* Bibliography::
 318@end menu
 319
 320@node QEMU compared to other emulators
 321@section QEMU compared to other emulators
 322
 323Like bochs [3], QEMU emulates an x86 CPU. But QEMU is much faster than
 324bochs as it uses dynamic compilation. Bochs is closely tied to x86 PC
 325emulation while QEMU can emulate several processors.
 326
 327Like Valgrind [2], QEMU does user space emulation and dynamic
 328translation. Valgrind is mainly a memory debugger while QEMU has no
 329support for it (QEMU could be used to detect out of bound memory
 330accesses as Valgrind, but it has no support to track uninitialised data
 331as Valgrind does). The Valgrind dynamic translator generates better code
 332than QEMU (in particular it does register allocation) but it is closely
 333tied to an x86 host and target and has no support for precise exceptions
 334and system emulation.
 335
 336EM86 [4] is the closest project to user space QEMU (and QEMU still uses
 337some of its code, in particular the ELF file loader). EM86 was limited
 338to an alpha host and used a proprietary and slow interpreter (the
 339interpreter part of the FX!32 Digital Win32 code translator [5]).
 340
 341TWIN [6] is a Windows API emulator like Wine. It is less accurate than
 342Wine but includes a protected mode x86 interpreter to launch x86 Windows
 343executables. Such an approach has greater potential because most of the
 344Windows API is executed natively but it is far more difficult to develop
 345because all the data structures and function parameters exchanged
 346between the API and the x86 code must be converted.
 347
 348User mode Linux [7] was the only solution before QEMU to launch a
 349Linux kernel as a process while not needing any host kernel
 350patches. However, user mode Linux requires heavy kernel patches while
 351QEMU accepts unpatched Linux kernels. The price to pay is that QEMU is
 352slower.
 353
 354The Plex86 [8] PC virtualizer is done in the same spirit as the now
 355obsolete qemu-fast system emulator. It requires a patched Linux kernel
 356to work (you cannot launch the same kernel on your PC), but the
 357patches are really small. As it is a PC virtualizer (no emulation is
 358done except for some privileged instructions), it has the potential of
 359being faster than QEMU. The downside is that a complicated (and
 360potentially unsafe) host kernel patch is needed.
 361
 362The commercial PC Virtualizers (VMWare [9], VirtualPC [10], TwoOStwo
 363[11]) are faster than QEMU, but they all need specific, proprietary
 364and potentially unsafe host drivers. Moreover, they are unable to
 365provide cycle exact simulation as an emulator can.
 366
 367VirtualBox [12], Xen [13] and KVM [14] are based on QEMU. QEMU-SystemC
 368[15] uses QEMU to simulate a system where some hardware devices are
 369developed in SystemC.
 370
 371@node Portable dynamic translation
 372@section Portable dynamic translation
 373
 374QEMU is a dynamic translator. When it first encounters a piece of code,
 375it converts it to the host instruction set. Usually dynamic translators
 376are very complicated and highly CPU dependent. QEMU uses some tricks
 377which make it relatively easily portable and simple while achieving good
 378performances.
 379
 380After the release of version 0.9.1, QEMU switched to a new method of
 381generating code, Tiny Code Generator or TCG. TCG relaxes the
 382dependency on the exact version of the compiler used. The basic idea
 383is to split every target instruction into a couple of RISC-like TCG
 384ops (see @code{target-i386/translate.c}). Some optimizations can be
 385performed at this stage, including liveness analysis and trivial
 386constant expression evaluation. TCG ops are then implemented in the
 387host CPU back end, also known as TCG target (see
 388@code{tcg/i386/tcg-target.c}). For more information, please take a
 389look at @code{tcg/README}.
 390
 391@node Condition code optimisations
 392@section Condition code optimisations
 393
 394Lazy evaluation of CPU condition codes (@code{EFLAGS} register on x86)
 395is important for CPUs where every instruction sets the condition
 396codes. It tends to be less important on conventional RISC systems
 397where condition codes are only updated when explicitly requested. On
 398Sparc64, costly update of both 32 and 64 bit condition codes can be
 399avoided with lazy evaluation.
 400
 401Instead of computing the condition codes after each x86 instruction,
 402QEMU just stores one operand (called @code{CC_SRC}), the result
 403(called @code{CC_DST}) and the type of operation (called
 404@code{CC_OP}). When the condition codes are needed, the condition
 405codes can be calculated using this information. In addition, an
 406optimized calculation can be performed for some instruction types like
 407conditional branches.
 408
 409@code{CC_OP} is almost never explicitly set in the generated code
 410because it is known at translation time.
 411
 412The lazy condition code evaluation is used on x86, m68k, cris and
 413Sparc. ARM uses a simplified variant for the N and Z flags.
 414
 415@node CPU state optimisations
 416@section CPU state optimisations
 417
 418The target CPUs have many internal states which change the way it
 419evaluates instructions. In order to achieve a good speed, the
 420translation phase considers that some state information of the virtual
 421CPU cannot change in it. The state is recorded in the Translation
 422Block (TB). If the state changes (e.g. privilege level), a new TB will
 423be generated and the previous TB won't be used anymore until the state
 424matches the state recorded in the previous TB. For example, if the SS,
 425DS and ES segments have a zero base, then the translator does not even
 426generate an addition for the segment base.
 427
 428[The FPU stack pointer register is not handled that way yet].
 429
 430@node Translation cache
 431@section Translation cache
 432
 433A 32 MByte cache holds the most recently used translations. For
 434simplicity, it is completely flushed when it is full. A translation unit
 435contains just a single basic block (a block of x86 instructions
 436terminated by a jump or by a virtual CPU state change which the
 437translator cannot deduce statically).
 438
 439@node Direct block chaining
 440@section Direct block chaining
 441
 442After each translated basic block is executed, QEMU uses the simulated
 443Program Counter (PC) and other cpu state informations (such as the CS
 444segment base value) to find the next basic block.
 445
 446In order to accelerate the most common cases where the new simulated PC
 447is known, QEMU can patch a basic block so that it jumps directly to the
 448next one.
 449
 450The most portable code uses an indirect jump. An indirect jump makes
 451it easier to make the jump target modification atomic. On some host
 452architectures (such as x86 or PowerPC), the @code{JUMP} opcode is
 453directly patched so that the block chaining has no overhead.
 454
 455@node Self-modifying code and translated code invalidation
 456@section Self-modifying code and translated code invalidation
 457
 458Self-modifying code is a special challenge in x86 emulation because no
 459instruction cache invalidation is signaled by the application when code
 460is modified.
 461
 462When translated code is generated for a basic block, the corresponding
 463host page is write protected if it is not already read-only. Then, if
 464a write access is done to the page, Linux raises a SEGV signal. QEMU
 465then invalidates all the translated code in the page and enables write
 466accesses to the page.
 467
 468Correct translated code invalidation is done efficiently by maintaining
 469a linked list of every translated block contained in a given page. Other
 470linked lists are also maintained to undo direct block chaining.
 471
 472On RISC targets, correctly written software uses memory barriers and
 473cache flushes, so some of the protection above would not be
 474necessary. However, QEMU still requires that the generated code always
 475matches the target instructions in memory in order to handle
 476exceptions correctly.
 477
 478@node Exception support
 479@section Exception support
 480
 481longjmp() is used when an exception such as division by zero is
 482encountered.
 483
 484The host SIGSEGV and SIGBUS signal handlers are used to get invalid
 485memory accesses. The simulated program counter is found by
 486retranslating the corresponding basic block and by looking where the
 487host program counter was at the exception point.
 488
 489The virtual CPU cannot retrieve the exact @code{EFLAGS} register because
 490in some cases it is not computed because of condition code
 491optimisations. It is not a big concern because the emulated code can
 492still be restarted in any cases.
 493
 494@node MMU emulation
 495@section MMU emulation
 496
 497For system emulation QEMU supports a soft MMU. In that mode, the MMU
 498virtual to physical address translation is done at every memory
 499access. QEMU uses an address translation cache to speed up the
 500translation.
 501
 502In order to avoid flushing the translated code each time the MMU
 503mappings change, QEMU uses a physically indexed translation cache. It
 504means that each basic block is indexed with its physical address.
 505
 506When MMU mappings change, only the chaining of the basic blocks is
 507reset (i.e. a basic block can no longer jump directly to another one).
 508
 509@node Device emulation
 510@section Device emulation
 511
 512Systems emulated by QEMU are organized by boards. At initialization
 513phase, each board instantiates a number of CPUs, devices, RAM and
 514ROM. Each device in turn can assign I/O ports or memory areas (for
 515MMIO) to its handlers. When the emulation starts, an access to the
 516ports or MMIO memory areas assigned to the device causes the
 517corresponding handler to be called.
 518
 519RAM and ROM are handled more optimally, only the offset to the host
 520memory needs to be added to the guest address.
 521
 522The video RAM of VGA and other display cards is special: it can be
 523read or written directly like RAM, but write accesses cause the memory
 524to be marked with VGA_DIRTY flag as well.
 525
 526QEMU supports some device classes like serial and parallel ports, USB,
 527drives and network devices, by providing APIs for easier connection to
 528the generic, higher level implementations. The API hides the
 529implementation details from the devices, like native device use or
 530advanced block device formats like QCOW.
 531
 532Usually the devices implement a reset method and register support for
 533saving and loading of the device state. The devices can also use
 534timers, especially together with the use of bottom halves (BHs).
 535
 536@node Hardware interrupts
 537@section Hardware interrupts
 538
 539In order to be faster, QEMU does not check at every basic block if a
 540hardware interrupt is pending. Instead, the user must asynchronously
 541call a specific function to tell that an interrupt is pending. This
 542function resets the chaining of the currently executing basic
 543block. It ensures that the execution will return soon in the main loop
 544of the CPU emulator. Then the main loop can test if the interrupt is
 545pending and handle it.
 546
 547@node User emulation specific details
 548@section User emulation specific details
 549
 550@subsection Linux system call translation
 551
 552QEMU includes a generic system call translator for Linux. It means that
 553the parameters of the system calls can be converted to fix the
 554endianness and 32/64 bit issues. The IOCTLs are converted with a generic
 555type description system (see @file{ioctls.h} and @file{thunk.c}).
 556
 557QEMU supports host CPUs which have pages bigger than 4KB. It records all
 558the mappings the process does and try to emulated the @code{mmap()}
 559system calls in cases where the host @code{mmap()} call would fail
 560because of bad page alignment.
 561
 562@subsection Linux signals
 563
 564Normal and real-time signals are queued along with their information
 565(@code{siginfo_t}) as it is done in the Linux kernel. Then an interrupt
 566request is done to the virtual CPU. When it is interrupted, one queued
 567signal is handled by generating a stack frame in the virtual CPU as the
 568Linux kernel does. The @code{sigreturn()} system call is emulated to return
 569from the virtual signal handler.
 570
 571Some signals (such as SIGALRM) directly come from the host. Other
 572signals are synthesized from the virtual CPU exceptions such as SIGFPE
 573when a division by zero is done (see @code{main.c:cpu_loop()}).
 574
 575The blocked signal mask is still handled by the host Linux kernel so
 576that most signal system calls can be redirected directly to the host
 577Linux kernel. Only the @code{sigaction()} and @code{sigreturn()} system
 578calls need to be fully emulated (see @file{signal.c}).
 579
 580@subsection clone() system call and threads
 581
 582The Linux clone() system call is usually used to create a thread. QEMU
 583uses the host clone() system call so that real host threads are created
 584for each emulated thread. One virtual CPU instance is created for each
 585thread.
 586
 587The virtual x86 CPU atomic operations are emulated with a global lock so
 588that their semantic is preserved.
 589
 590Note that currently there are still some locking issues in QEMU. In
 591particular, the translated cache flush is not protected yet against
 592reentrancy.
 593
 594@subsection Self-virtualization
 595
 596QEMU was conceived so that ultimately it can emulate itself. Although
 597it is not very useful, it is an important test to show the power of the
 598emulator.
 599
 600Achieving self-virtualization is not easy because there may be address
 601space conflicts. QEMU user emulators solve this problem by being an
 602executable ELF shared object as the ld-linux.so ELF interpreter. That
 603way, it can be relocated at load time.
 604
 605@node Bibliography
 606@section Bibliography
 607
 608@table @asis
 609
 610@item [1]
 611@url{http://citeseer.nj.nec.com/piumarta98optimizing.html}, Optimizing
 612direct threaded code by selective inlining (1998) by Ian Piumarta, Fabio
 613Riccardi.
 614
 615@item [2]
 616@url{http://developer.kde.org/~sewardj/}, Valgrind, an open-source
 617memory debugger for x86-GNU/Linux, by Julian Seward.
 618
 619@item [3]
 620@url{http://bochs.sourceforge.net/}, the Bochs IA-32 Emulator Project,
 621by Kevin Lawton et al.
 622
 623@item [4]
 624@url{http://www.cs.rose-hulman.edu/~donaldlf/em86/index.html}, the EM86
 625x86 emulator on Alpha-Linux.
 626
 627@item [5]
 628@url{http://www.usenix.org/publications/library/proceedings/usenix-nt97/@/full_papers/chernoff/chernoff.pdf},
 629DIGITAL FX!32: Running 32-Bit x86 Applications on Alpha NT, by Anton
 630Chernoff and Ray Hookway.
 631
 632@item [6]
 633@url{http://www.willows.com/}, Windows API library emulation from
 634Willows Software.
 635
 636@item [7]
 637@url{http://user-mode-linux.sourceforge.net/},
 638The User-mode Linux Kernel.
 639
 640@item [8]
 641@url{http://www.plex86.org/},
 642The new Plex86 project.
 643
 644@item [9]
 645@url{http://www.vmware.com/},
 646The VMWare PC virtualizer.
 647
 648@item [10]
 649@url{http://www.microsoft.com/windowsxp/virtualpc/},
 650The VirtualPC PC virtualizer.
 651
 652@item [11]
 653@url{http://www.twoostwo.org/},
 654The TwoOStwo PC virtualizer.
 655
 656@item [12]
 657@url{http://virtualbox.org/},
 658The VirtualBox PC virtualizer.
 659
 660@item [13]
 661@url{http://www.xen.org/},
 662The Xen hypervisor.
 663
 664@item [14]
 665@url{http://kvm.qumranet.com/kvmwiki/Front_Page},
 666Kernel Based Virtual Machine (KVM).
 667
 668@item [15]
 669@url{http://www.greensocs.com/projects/QEMUSystemC},
 670QEMU-SystemC, a hardware co-simulator.
 671
 672@end table
 673
 674@node Regression Tests
 675@chapter Regression Tests
 676
 677In the directory @file{tests/}, various interesting testing programs
 678are available. They are used for regression testing.
 679
 680@menu
 681* test-i386::
 682* linux-test::
 683@end menu
 684
 685@node test-i386
 686@section @file{test-i386}
 687
 688This program executes most of the 16 bit and 32 bit x86 instructions and
 689generates a text output. It can be compared with the output obtained with
 690a real CPU or another emulator. The target @code{make test} runs this
 691program and a @code{diff} on the generated output.
 692
 693The Linux system call @code{modify_ldt()} is used to create x86 selectors
 694to test some 16 bit addressing and 32 bit with segmentation cases.
 695
 696The Linux system call @code{vm86()} is used to test vm86 emulation.
 697
 698Various exceptions are raised to test most of the x86 user space
 699exception reporting.
 700
 701@node linux-test
 702@section @file{linux-test}
 703
 704This program tests various Linux system calls. It is used to verify
 705that the system call parameters are correctly converted between target
 706and host CPUs.
 707
 708@node Index
 709@chapter Index
 710@printindex cp
 711
 712@bye
 713