X-Git-Url: https://repo.jachan.dev/qemu.git/blobdiff_plain/c3ce5a235741cb027b1328288ddec06470254813..1b5c15cebd5cc7595337a8a7cf14f6a707605e99:/qemu-tech.texi diff --git a/qemu-tech.texi b/qemu-tech.texi index 75ceea408c..0380de77b6 100644 --- a/qemu-tech.texi +++ b/qemu-tech.texi @@ -1,57 +1,25 @@ -\input texinfo @c -*- texinfo -*- -@c %**start of header -@setfilename qemu-tech.info - -@documentlanguage en -@documentencoding UTF-8 - -@settitle QEMU Internals -@exampleindent 0 -@paragraphindent 0 -@c %**end of header - -@ifinfo -@direntry -* QEMU Internals: (qemu-tech). The QEMU Emulator Internals. -@end direntry -@end ifinfo - -@iftex -@titlepage -@sp 7 -@center @titlefont{QEMU Internals} -@sp 3 -@end titlepage -@end iftex - -@ifnottex -@node Top -@top +@node Implementation notes +@appendix Implementation notes @menu -* Introduction:: -* QEMU Internals:: -* Regression Tests:: +* CPU emulation:: +* Managed start up options:: @end menu -@end ifnottex -@contents - -@node Introduction -@chapter Introduction +@node CPU emulation +@section CPU emulation @menu -* intro_x86_emulation:: x86 and x86-64 emulation -* intro_arm_emulation:: ARM emulation -* intro_mips_emulation:: MIPS emulation -* intro_ppc_emulation:: PowerPC emulation -* intro_sparc_emulation:: Sparc32 and Sparc64 emulation -* intro_xtensa_emulation:: Xtensa emulation -* intro_other_emulation:: Other CPU emulation +* x86:: x86 and x86-64 emulation +* ARM:: ARM emulation +* MIPS:: MIPS emulation +* PPC:: PowerPC emulation +* SPARC:: Sparc32 and Sparc64 emulation +* Xtensa:: Xtensa emulation @end menu -@node intro_x86_emulation -@section x86 and x86-64 emulation +@node x86 +@subsection x86 and x86-64 emulation QEMU x86 target features: @@ -85,8 +53,8 @@ normal use. @end itemize -@node intro_arm_emulation -@section ARM emulation +@node ARM +@subsection ARM emulation @itemize @@ -98,8 +66,8 @@ normal use. @end itemize -@node intro_mips_emulation -@section MIPS emulation +@node MIPS +@subsection MIPS emulation @itemize @@ -125,8 +93,8 @@ Current QEMU limitations: @end itemize -@node intro_ppc_emulation -@section PowerPC emulation +@node PPC +@subsection PowerPC emulation @itemize @@ -137,8 +105,8 @@ FPU and MMU. @end itemize -@node intro_sparc_emulation -@section Sparc32 and Sparc64 emulation +@node SPARC +@subsection Sparc32 and Sparc64 emulation @itemize @@ -165,8 +133,8 @@ Current QEMU limitations: @end itemize -@node intro_xtensa_emulation -@section Xtensa emulation +@node Xtensa +@subsection Xtensa emulation @itemize @@ -190,383 +158,38 @@ may be created from overlay with minimal amount of hand-written code. @end itemize -@node intro_other_emulation -@section Other CPU emulation - -In addition to the above, QEMU supports emulation of other CPUs with -varying levels of success. These are: - -@itemize - -@item -Alpha -@item -CRIS -@item -M68k -@item -SH4 -@end itemize - -@node QEMU Internals -@chapter QEMU Internals - -@menu -* QEMU compared to other emulators:: -* Portable dynamic translation:: -* CPU state optimisations:: -* Translation cache:: -* Direct block chaining:: -* Self-modifying code and translated code invalidation:: -* Exception support:: -* MMU emulation:: -* Device emulation:: -* Hardware interrupts:: -* User emulation specific details:: -* Bibliography:: -@end menu - -@node QEMU compared to other emulators -@section QEMU compared to other emulators - -Like bochs [1], QEMU emulates an x86 CPU. But QEMU is much faster than -bochs as it uses dynamic compilation. Bochs is closely tied to x86 PC -emulation while QEMU can emulate several processors. - -Like Valgrind [2], QEMU does user space emulation and dynamic -translation. Valgrind is mainly a memory debugger while QEMU has no -support for it (QEMU could be used to detect out of bound memory -accesses as Valgrind, but it has no support to track uninitialised data -as Valgrind does). The Valgrind dynamic translator generates better code -than QEMU (in particular it does register allocation) but it is closely -tied to an x86 host and target and has no support for precise exceptions -and system emulation. - -EM86 [3] is the closest project to user space QEMU (and QEMU still uses -some of its code, in particular the ELF file loader). EM86 was limited -to an alpha host and used a proprietary and slow interpreter (the -interpreter part of the FX!32 Digital Win32 code translator [4]). - -TWIN from Willows Software was a Windows API emulator like Wine. It is less -accurate than Wine but includes a protected mode x86 interpreter to launch -x86 Windows executables. Such an approach has greater potential because most -of the Windows API is executed natively but it is far more difficult to -develop because all the data structures and function parameters exchanged -between the API and the x86 code must be converted. - -User mode Linux [5] was the only solution before QEMU to launch a -Linux kernel as a process while not needing any host kernel -patches. However, user mode Linux requires heavy kernel patches while -QEMU accepts unpatched Linux kernels. The price to pay is that QEMU is -slower. - -The Plex86 [6] PC virtualizer is done in the same spirit as the now -obsolete qemu-fast system emulator. It requires a patched Linux kernel -to work (you cannot launch the same kernel on your PC), but the -patches are really small. As it is a PC virtualizer (no emulation is -done except for some privileged instructions), it has the potential of -being faster than QEMU. The downside is that a complicated (and -potentially unsafe) host kernel patch is needed. - -The commercial PC Virtualizers (VMWare [7], VirtualPC [8]) are faster -than QEMU (without virtualization), but they all need specific, proprietary -and potentially unsafe host drivers. Moreover, they are unable to -provide cycle exact simulation as an emulator can. - -VirtualBox [9], Xen [10] and KVM [11] are based on QEMU. QEMU-SystemC -[12] uses QEMU to simulate a system where some hardware devices are -developed in SystemC. - -@node Portable dynamic translation -@section Portable dynamic translation - -QEMU is a dynamic translator. When it first encounters a piece of code, -it converts it to the host instruction set. Usually dynamic translators -are very complicated and highly CPU dependent. QEMU uses some tricks -which make it relatively easily portable and simple while achieving good -performances. - -QEMU's dynamic translation backend is called TCG, for "Tiny Code -Generator". For more information, please take a look at @code{tcg/README}. - -@node CPU state optimisations -@section CPU state optimisations - -The target CPUs have many internal states which change the way it -evaluates instructions. In order to achieve a good speed, the -translation phase considers that some state information of the virtual -CPU cannot change in it. The state is recorded in the Translation -Block (TB). If the state changes (e.g. privilege level), a new TB will -be generated and the previous TB won't be used anymore until the state -matches the state recorded in the previous TB. For example, if the SS, -DS and ES segments have a zero base, then the translator does not even -generate an addition for the segment base. - -[The FPU stack pointer register is not handled that way yet]. - -@node Translation cache -@section Translation cache - -A 32 MByte cache holds the most recently used translations. For -simplicity, it is completely flushed when it is full. A translation unit -contains just a single basic block (a block of x86 instructions -terminated by a jump or by a virtual CPU state change which the -translator cannot deduce statically). - -@node Direct block chaining -@section Direct block chaining - -After each translated basic block is executed, QEMU uses the simulated -Program Counter (PC) and other cpu state information (such as the CS -segment base value) to find the next basic block. - -In order to accelerate the most common cases where the new simulated PC -is known, QEMU can patch a basic block so that it jumps directly to the -next one. - -The most portable code uses an indirect jump. An indirect jump makes -it easier to make the jump target modification atomic. On some host -architectures (such as x86 or PowerPC), the @code{JUMP} opcode is -directly patched so that the block chaining has no overhead. - -@node Self-modifying code and translated code invalidation -@section Self-modifying code and translated code invalidation - -Self-modifying code is a special challenge in x86 emulation because no -instruction cache invalidation is signaled by the application when code -is modified. - -When translated code is generated for a basic block, the corresponding -host page is write protected if it is not already read-only. Then, if -a write access is done to the page, Linux raises a SEGV signal. QEMU -then invalidates all the translated code in the page and enables write -accesses to the page. - -Correct translated code invalidation is done efficiently by maintaining -a linked list of every translated block contained in a given page. Other -linked lists are also maintained to undo direct block chaining. - -On RISC targets, correctly written software uses memory barriers and -cache flushes, so some of the protection above would not be -necessary. However, QEMU still requires that the generated code always -matches the target instructions in memory in order to handle -exceptions correctly. - -@node Exception support -@section Exception support - -longjmp() is used when an exception such as division by zero is -encountered. - -The host SIGSEGV and SIGBUS signal handlers are used to get invalid -memory accesses. The simulated program counter is found by -retranslating the corresponding basic block and by looking where the -host program counter was at the exception point. - -The virtual CPU cannot retrieve the exact @code{EFLAGS} register because -in some cases it is not computed because of condition code -optimisations. It is not a big concern because the emulated code can -still be restarted in any cases. - -@node MMU emulation -@section MMU emulation - -For system emulation QEMU supports a soft MMU. In that mode, the MMU -virtual to physical address translation is done at every memory -access. QEMU uses an address translation cache to speed up the -translation. - -In order to avoid flushing the translated code each time the MMU -mappings change, QEMU uses a physically indexed translation cache. It -means that each basic block is indexed with its physical address. - -When MMU mappings change, only the chaining of the basic blocks is -reset (i.e. a basic block can no longer jump directly to another one). - -@node Device emulation -@section Device emulation - -Systems emulated by QEMU are organized by boards. At initialization -phase, each board instantiates a number of CPUs, devices, RAM and -ROM. Each device in turn can assign I/O ports or memory areas (for -MMIO) to its handlers. When the emulation starts, an access to the -ports or MMIO memory areas assigned to the device causes the -corresponding handler to be called. - -RAM and ROM are handled more optimally, only the offset to the host -memory needs to be added to the guest address. - -The video RAM of VGA and other display cards is special: it can be -read or written directly like RAM, but write accesses cause the memory -to be marked with VGA_DIRTY flag as well. - -QEMU supports some device classes like serial and parallel ports, USB, -drives and network devices, by providing APIs for easier connection to -the generic, higher level implementations. The API hides the -implementation details from the devices, like native device use or -advanced block device formats like QCOW. - -Usually the devices implement a reset method and register support for -saving and loading of the device state. The devices can also use -timers, especially together with the use of bottom halves (BHs). - -@node Hardware interrupts -@section Hardware interrupts - -In order to be faster, QEMU does not check at every basic block if a -hardware interrupt is pending. Instead, the user must asynchronously -call a specific function to tell that an interrupt is pending. This -function resets the chaining of the currently executing basic -block. It ensures that the execution will return soon in the main loop -of the CPU emulator. Then the main loop can test if the interrupt is -pending and handle it. - -@node User emulation specific details -@section User emulation specific details - -@subsection Linux system call translation - -QEMU includes a generic system call translator for Linux. It means that -the parameters of the system calls can be converted to fix the -endianness and 32/64 bit issues. The IOCTLs are converted with a generic -type description system (see @file{ioctls.h} and @file{thunk.c}). - -QEMU supports host CPUs which have pages bigger than 4KB. It records all -the mappings the process does and try to emulated the @code{mmap()} -system calls in cases where the host @code{mmap()} call would fail -because of bad page alignment. - -@subsection Linux signals - -Normal and real-time signals are queued along with their information -(@code{siginfo_t}) as it is done in the Linux kernel. Then an interrupt -request is done to the virtual CPU. When it is interrupted, one queued -signal is handled by generating a stack frame in the virtual CPU as the -Linux kernel does. The @code{sigreturn()} system call is emulated to return -from the virtual signal handler. - -Some signals (such as SIGALRM) directly come from the host. Other -signals are synthesized from the virtual CPU exceptions such as SIGFPE -when a division by zero is done (see @code{main.c:cpu_loop()}). - -The blocked signal mask is still handled by the host Linux kernel so -that most signal system calls can be redirected directly to the host -Linux kernel. Only the @code{sigaction()} and @code{sigreturn()} system -calls need to be fully emulated (see @file{signal.c}). - -@subsection clone() system call and threads - -The Linux clone() system call is usually used to create a thread. QEMU -uses the host clone() system call so that real host threads are created -for each emulated thread. One virtual CPU instance is created for each -thread. - -The virtual x86 CPU atomic operations are emulated with a global lock so -that their semantic is preserved. - -Note that currently there are still some locking issues in QEMU. In -particular, the translated cache flush is not protected yet against -reentrancy. - -@subsection Self-virtualization - -QEMU was conceived so that ultimately it can emulate itself. Although -it is not very useful, it is an important test to show the power of the -emulator. - -Achieving self-virtualization is not easy because there may be address -space conflicts. QEMU user emulators solve this problem by being an -executable ELF shared object as the ld-linux.so ELF interpreter. That -way, it can be relocated at load time. - -@node Bibliography -@section Bibliography +@node Managed start up options +@section Managed start up options +In system mode emulation, it's possible to create a VM in a paused state using +the -S command line option. In this state the machine is completely initialized +according to command line options and ready to execute VM code but VCPU threads +are not executing any code. The VM state in this paused state depends on the way +QEMU was started. It could be in: @table @asis - -@item [1] -@url{http://bochs.sourceforge.net/}, the Bochs IA-32 Emulator Project, -by Kevin Lawton et al. - -@item [2] -@url{http://www.valgrind.org/}, Valgrind, an open-source memory debugger -for GNU/Linux. - -@item [3] -@url{http://ftp.dreamtime.org/pub/linux/Linux-Alpha/em86/v0.2/docs/em86.html}, -the EM86 x86 emulator on Alpha-Linux. - -@item [4] -@url{http://www.usenix.org/publications/library/proceedings/usenix-nt97/@/full_papers/chernoff/chernoff.pdf}, -DIGITAL FX!32: Running 32-Bit x86 Applications on Alpha NT, by Anton -Chernoff and Ray Hookway. - -@item [5] -@url{http://user-mode-linux.sourceforge.net/}, -The User-mode Linux Kernel. - -@item [6] -@url{http://www.plex86.org/}, -The new Plex86 project. - -@item [7] -@url{http://www.vmware.com/}, -The VMWare PC virtualizer. - -@item [8] -@url{https://www.microsoft.com/download/details.aspx?id=3702}, -The VirtualPC PC virtualizer. - -@item [9] -@url{http://virtualbox.org/}, -The VirtualBox PC virtualizer. - -@item [10] -@url{http://www.xen.org/}, -The Xen hypervisor. - -@item [11] -@url{http://www.linux-kvm.org/}, -Kernel Based Virtual Machine (KVM). - -@item [12] -@url{http://www.greensocs.com/projects/QEMUSystemC}, -QEMU-SystemC, a hardware co-simulator. - +@item initial state (after reset/power on state) +@item with direct kernel loading, the initial state could be amended to execute +code loaded by QEMU in the VM's RAM and with incoming migration +@item with incoming migration, initial state will by amended with the migrated +machine state after migration completes. @end table -@node Regression Tests -@chapter Regression Tests - -In the directory @file{tests/}, various interesting testing programs -are available. They are used for regression testing. - -@menu -* test-i386:: -* linux-test:: -@end menu - -@node test-i386 -@section @file{test-i386} - -This program executes most of the 16 bit and 32 bit x86 instructions and -generates a text output. It can be compared with the output obtained with -a real CPU or another emulator. The target @code{make test} runs this -program and a @code{diff} on the generated output. - -The Linux system call @code{modify_ldt()} is used to create x86 selectors -to test some 16 bit addressing and 32 bit with segmentation cases. - -The Linux system call @code{vm86()} is used to test vm86 emulation. - -Various exceptions are raised to test most of the x86 user space -exception reporting. - -@node linux-test -@section @file{linux-test} - -This program tests various Linux system calls. It is used to verify -that the system call parameters are correctly converted between target -and host CPUs. - -@bye +This paused state is typically used by users to query machine state and/or +additionally configure the machine (by hotplugging devices) in runtime before +allowing VM code to run. + +However, at the -S pause point, it's impossible to configure options that affect +initial VM creation (like: -smp/-m/-numa ...) or cold plug devices. The +experimental --preconfig command line option allows pausing QEMU +before the initial VM creation, in a ``preconfig'' state, where additional +queries and configuration can be performed via QMP before moving on to +the resulting configuration startup. In the preconfig state, QEMU only allows +a limited set of commands over the QMP monitor, where the commands do not +depend on an initialized machine, including but not limited to: +@table @asis +@item qmp_capabilities +@item query-qmp-schema +@item query-commands +@item query-status +@item x-exit-preconfig +@end table