diff options
Diffstat (limited to 'qemu-doc.texi')
| -rw-r--r-- | qemu-doc.texi | 136 |
1 files changed, 88 insertions, 48 deletions
diff --git a/qemu-doc.texi b/qemu-doc.texi index 2e324ca988..f63a17ac06 100644 --- a/qemu-doc.texi +++ b/qemu-doc.texi @@ -10,11 +10,11 @@ @chapter Introduction QEMU is an x86 processor emulator. Its purpose is to run x86 Linux -processes on non-x86 Linux architectures such as PowerPC or ARM. By -using dynamic translation it achieves a reasonnable speed while being -easy to port on new host CPUs. Its main goal is to be able to launch the -@code{Wine} Windows API emulator (@url{http://www.winehq.org}) on -non-x86 CPUs. +processes on non-x86 Linux architectures such as PowerPC. By using +dynamic translation it achieves a reasonnable speed while being easy to +port on new host CPUs. Its main goal is to be able to launch the +@code{Wine} Windows API emulator (@url{http://www.winehq.org}) or +@code{DOSEMU} (@url{http://www.dosemu.org}) on non-x86 CPUs. QEMU features: @@ -22,21 +22,26 @@ QEMU features: @item User space only x86 emulator. -@item Currently ported on i386, PowerPC and S390. +@item Currently ported on i386, PowerPC. Work in progress for S390, Alpha and Sparc. @item Using dynamic translation to native code for reasonnable speed. @item The virtual x86 CPU supports 16 bit and 32 bit addressing with segmentation. -User space LDT and GDT are emulated. VM86 mode is also supported -(experimental). +User space LDT and GDT are emulated. VM86 mode is also supported. @item Generic Linux system call converter, including most ioctls. @item clone() emulation using native CPU clone() to use Linux scheduler for threads. -@item Accurate signal handling by remapping host signals to virtual x86 signals. +@item Accurate signal handling by remapping host signals to virtual x86 signals. -@item QEMU can emulate itself on x86 (experimental). +@item Precise user space x86 exceptions. + +@item Self-modifying code support. + +@item Support of host page sizes bigger than 4KB. + +@item QEMU can emulate itself on x86. @item The virtual x86 CPU is a library (@code{libqemu}) which can be used in other projects. @@ -46,19 +51,15 @@ It can be used to test other x86 virtual CPUs. @end itemize -Current QEMU Limitations: +Current QEMU limitations: @itemize -@item Not all x86 exceptions are precise (yet). [Very few programs need that]. - -@item No support for self-modifying code (yet). [Very few programs need that, a notable exception is QEMU itself !]. - @item No SSE/MMX support (yet). @item No x86-64 support. -@item Some Linux syscalls are missing. +@item IPC syscalls are missing. @item The x86 segment limits and access rights are not tested at every memory access (and will never be to have good performances). @@ -119,7 +120,7 @@ qemu /usr/local/qemu-i386/bin/qemu-i386 /usr/local/qemu-i386/bin/ls-i386 @end itemize -@section Wine launch (Currently only tested when emulating x86 on x86) +@section Wine launch @itemize @@ -152,17 +153,24 @@ qemu /usr/local/qemu-i386/wine/bin/wine /usr/local/qemu-i386/wine/c/Program\ Fil usage: qemu [-h] [-d] [-L path] [-s size] program [arguments...] @end example -@table @samp +@table @option @item -h Print the help -@item -d -Activate log (logfile=/tmp/qemu.log) @item -L path Set the x86 elf interpreter prefix (default=/usr/local/qemu-i386) @item -s size Set the x86 stack size in bytes (default=524288) @end table +Debug options: + +@table @option +@item -d +Activate log (logfile=/tmp/qemu.log) +@item -p pagesize +Act as if the host page size was 'pagesize' bytes +@end table + @chapter QEMU Internals @section QEMU compared to other emulators @@ -265,17 +273,59 @@ 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). -[Currently, the translated code is not patched if it jumps to another -translated code]. +@section Direct block chaining + +After each translated basic block is executed, QEMU uses the simulated +Program Counter (PC) and other cpu state informations (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 +architectures (such as PowerPC), the @code{JUMP} opcode is directly +patched so that the block chaining has no overhead. + +@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 (with the +system call @code{mprotect()}). 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. + +Althought the overhead of doing @code{mprotect()} calls is important, +most MSDOS programs can be emulated at reasonnable speed with QEMU and +DOSEMU. + +Note that QEMU also invalidates pages of translated code when it detects +that memory mappings are modified with @code{mmap()} or @code{munmap()}. @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. +encountered. -[Currently, the virtual CPU cannot retrieve the exact CPU state in some -exceptions, although it could except for the @code{EFLAGS} register]. +The host SIGSEGV and SIGBUS signal handlers are used to get invalid +memory accesses. The exact CPU state can be retrieved because all the +x86 registers are stored in fixed host registers. 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. @section Linux system call translation @@ -284,6 +334,11 @@ 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. + @section Linux signals Normal and real-time signals are queued along with their information @@ -312,6 +367,10 @@ 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. + @section Self-virtualization QEMU was conceived so that ultimately it can emulate itself. Althought @@ -323,10 +382,6 @@ space conflicts. QEMU solves 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. -Since self-modifying code is not supported yet, QEMU cannot emulate -itself in case of translation cache flush. This limitation will be -suppressed soon. - @section Bibliography @table @asis @@ -379,19 +434,10 @@ 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. -@section @file{testsig} - -This program tests various signal cases, including SIGFPE, SIGSEGV and -SIGILL. - -@section @file{testclone} +The Linux system call @code{vm86()} is used to test vm86 emulation. -Tests the @code{clone()} system call (basic test). - -@section @file{testthread} - -Tests the glibc threads (more complicated than @code{clone()} because signals -are also used). +Various exceptions are raised to test most of the x86 user space +exception reporting. @section @file{sha1} @@ -399,9 +445,3 @@ It is a simple benchmark. Care must be taken to interpret the results because it mostly tests the ability of the virtual CPU to optimize the @code{rol} x86 instruction and the condition code computations. -@section @file{runcom} - -A very simple MSDOS emulator to test the Linux vm86() system call -emulation. The excellent 54 byte @file{pi_10.com} PI number calculator -can be launched with it. @file{pi_10.com} was written by Bertram -Felgenhauer (more information at @url{http://www.boo.net/~jasonp/pipage.html}). |