Revision 386405f7
b/qemu-doc.texi | ||
---|---|---|
1 |
\input texinfo @c -*- texinfo -*- |
|
2 |
|
|
3 |
@settitle QEMU x86 Emulator Reference Documentation |
|
4 |
@titlepage |
|
5 |
@sp 7 |
|
6 |
@center @titlefont{QEMU x86 Emulator Reference Documentation} |
|
7 |
@sp 3 |
|
8 |
@end titlepage |
|
9 |
|
|
10 |
@chapter Introduction |
|
11 |
|
|
12 |
QEMU is an x86 processor emulator. Its purpose is to run x86 Linux |
|
13 |
processes on non-x86 Linux architectures such as PowerPC or ARM. By |
|
14 |
using dynamic translation it achieves a reasonnable speed while being |
|
15 |
easy to port on new host CPUs. An obviously interesting x86 only process |
|
16 |
is 'wine' (Windows emulation). |
|
17 |
|
|
18 |
QEMU features: |
|
19 |
|
|
20 |
@itemize |
|
21 |
|
|
22 |
@item User space only x86 emulator. |
|
23 |
|
|
24 |
@item Currently ported on i386 and PowerPC. |
|
25 |
|
|
26 |
@item Using dynamic translation for reasonnable speed. |
|
27 |
|
|
28 |
@item The virtual x86 CPU supports 16 bit and 32 bit addressing with segmentation. |
|
29 |
User space LDT and GDT are emulated. |
|
30 |
|
|
31 |
@item Generic Linux system call converter, including most ioctls. |
|
32 |
|
|
33 |
@item clone() emulation using native CPU clone() to use Linux scheduler for threads. |
|
34 |
|
|
35 |
@item Accurate signal handling by remapping host signals to virtual x86 signals. |
|
36 |
|
|
37 |
@item The virtual x86 CPU is a library (@code{libqemu}) which can be used |
|
38 |
in other projects. |
|
39 |
|
|
40 |
@item An extensive Linux x86 CPU test program is included @file{tests/test-i386}. |
|
41 |
It can be used to test other x86 virtual CPUs. |
|
42 |
|
|
43 |
@end itemize |
|
44 |
|
|
45 |
Current QEMU Limitations: |
|
46 |
|
|
47 |
@itemize |
|
48 |
|
|
49 |
@item Not all x86 exceptions are precise (yet). [Very few programs need that]. |
|
50 |
|
|
51 |
@item Not self virtualizable (yet). [You cannot launch qemu with qemu on the same CPU]. |
|
52 |
|
|
53 |
@item No support for self modifying code (yet). [Very few programs need that, a notable exception is QEMU itself !]. |
|
54 |
|
|
55 |
@item No VM86 mode (yet), althought the virtual |
|
56 |
CPU has support for most of it. [VM86 support is useful to launch old 16 |
|
57 |
bit DOS programs with dosemu or wine]. |
|
58 |
|
|
59 |
@item No SSE/MMX support (yet). |
|
60 |
|
|
61 |
@item No x86-64 support. |
|
62 |
|
|
63 |
@item Some Linux syscalls are missing. |
|
64 |
|
|
65 |
@item The x86 segment limits and access rights are not tested at every |
|
66 |
memory access (and will never be to have good performances). |
|
67 |
|
|
68 |
@item On non x86 host CPUs, @code{double}s are used instead of the non standard |
|
69 |
10 byte @code{long double}s of x86 for floating point emulation to get |
|
70 |
maximum performances. |
|
71 |
|
|
72 |
@end itemize |
|
73 |
|
|
74 |
@chapter Invocation |
|
75 |
|
|
76 |
In order to launch a Linux process, QEMU needs the process executable |
|
77 |
itself and all the target (x86) dynamic libraries used by it. Currently, |
|
78 |
QEMU is not distributed with the necessary packages so that you can test |
|
79 |
it easily on non x86 CPUs. |
|
80 |
|
|
81 |
However, the statically x86 binary 'tests/hello' can be used to do a |
|
82 |
first test: |
|
83 |
|
|
84 |
@example |
|
85 |
qemu tests/hello |
|
86 |
@end example |
|
87 |
|
|
88 |
@code{Hello world} should be printed on the terminal. |
|
89 |
|
|
90 |
If you are testing it on a x86 CPU, then you can test it on any process: |
|
91 |
|
|
92 |
@example |
|
93 |
qemu /bin/ls -l |
|
94 |
@end example |
|
95 |
|
|
96 |
@chapter QEMU Internals |
|
97 |
|
|
98 |
@section QEMU compared to other emulators |
|
99 |
|
|
100 |
Unlike bochs [3], QEMU emulates only a user space x86 CPU. It means that |
|
101 |
you cannot launch an operating system with it. The benefit is that it is |
|
102 |
simpler and faster due to the fact that some of the low level CPU state |
|
103 |
can be ignored (in particular, no virtual memory needs to be emulated). |
|
104 |
|
|
105 |
Like Valgrind [2], QEMU does user space emulation and dynamic |
|
106 |
translation. Valgrind is mainly a memory debugger while QEMU has no |
|
107 |
support for it (QEMU could be used to detect out of bound memory accesses |
|
108 |
as Valgrind, but it has no support to track uninitialised data as |
|
109 |
Valgrind does). Valgrind dynamic translator generates better code than |
|
110 |
QEMU (in particular it does register allocation) but it is closely tied |
|
111 |
to an x86 host. |
|
112 |
|
|
113 |
EM86 [4] is the closest project to QEMU (and QEMU still uses some of its |
|
114 |
code, in particular the ELF file loader). EM86 was limited to an alpha |
|
115 |
host and used a proprietary and slow interpreter (the interpreter part |
|
116 |
of the FX!32 Digital Win32 code translator [5]). |
|
117 |
|
|
118 |
@section Portable dynamic translation |
|
119 |
|
|
120 |
QEMU is a dynamic translator. When it first encounters a piece of code, |
|
121 |
it converts it to the host instruction set. Usually dynamic translators |
|
122 |
are very complicated and highly CPU dependant. QEMU uses some tricks |
|
123 |
which make it relatively easily portable and simple while achieving good |
|
124 |
performances. |
|
125 |
|
|
126 |
The basic idea is to split every x86 instruction into fewer simpler |
|
127 |
instructions. Each simple instruction is implemented by a piece of C |
|
128 |
code (see @file{op-i386.c}). Then a compile time tool (@file{dyngen}) |
|
129 |
takes the corresponding object file (@file{op-i386.o}) to generate a |
|
130 |
dynamic code generator which concatenates the simple instructions to |
|
131 |
build a function (see @file{op-i386.h:dyngen_code()}). |
|
132 |
|
|
133 |
In essence, the process is similar to [1], but more work is done at |
|
134 |
compile time. |
|
135 |
|
|
136 |
A key idea to get optimal performances is that constant parameters can |
|
137 |
be passed to the simple operations. For that purpose, dummy ELF |
|
138 |
relocations are generated with gcc for each constant parameter. Then, |
|
139 |
the tool (@file{dyngen}) can locate the relocations and generate the |
|
140 |
appriopriate C code to resolve them when building the dynamic code. |
|
141 |
|
|
142 |
That way, QEMU is no more difficult to port than a dynamic linker. |
|
143 |
|
|
144 |
To go even faster, GCC static register variables are used to keep the |
|
145 |
state of the virtual CPU. |
|
146 |
|
|
147 |
@section Register allocation |
|
148 |
|
|
149 |
Since QEMU uses fixed simple instructions, no efficient register |
|
150 |
allocation can be done. However, because RISC CPUs have a lot of |
|
151 |
register, most of the virtual CPU state can be put in registers without |
|
152 |
doing complicated register allocation. |
|
153 |
|
|
154 |
@section Condition code optimisations |
|
155 |
|
|
156 |
Good CPU condition codes emulation (@code{EFLAGS} register on x86) is a |
|
157 |
critical point to get good performances. QEMU uses lazy condition code |
|
158 |
evaluation: instead of computing the condition codes after each x86 |
|
159 |
instruction, it store justs one operand (called @code{CC_CRC}), the |
|
160 |
result (called @code{CC_DST}) and the type of operation (called |
|
161 |
@code{CC_OP}). |
|
162 |
|
|
163 |
@code{CC_OP} is almost never explicitely set in the generated code |
|
164 |
because it is known at translation time. |
|
165 |
|
|
166 |
In order to increase performances, a backward pass is performed on the |
|
167 |
generated simple instructions (see |
|
168 |
@code{translate-i386.c:optimize_flags()}). When it can be proved that |
|
169 |
the condition codes are not needed by the next instructions, no |
|
170 |
condition codes are computed at all. |
|
171 |
|
|
172 |
@section Translation CPU state optimisations |
|
173 |
|
|
174 |
The x86 CPU has many internal states which change the way it evaluates |
|
175 |
instructions. In order to achieve a good speed, the translation phase |
|
176 |
considers that some state information of the virtual x86 CPU cannot |
|
177 |
change in it. For example, if the SS, DS and ES segments have a zero |
|
178 |
base, then the translator does not even generate an addition for the |
|
179 |
segment base. |
|
180 |
|
|
181 |
[The FPU stack pointer register is not handled that way yet]. |
|
182 |
|
|
183 |
@section Translation cache |
|
184 |
|
|
185 |
A 2MByte cache holds the most recently used translations. For |
|
186 |
simplicity, it is completely flushed when it is full. A translation unit |
|
187 |
contains just a single basic block (a block of x86 instructions |
|
188 |
terminated by a jump or by a virtual CPU state change which the |
|
189 |
translator cannot deduce statically). |
|
190 |
|
|
191 |
[Currently, the translated code is not patched if it jumps to another |
|
192 |
translated code]. |
|
193 |
|
|
194 |
@section Exception support |
|
195 |
|
|
196 |
longjmp() is used when an exception such as division by zero is |
|
197 |
encountered. The host SIGSEGV and SIGBUS signal handlers are used to get |
|
198 |
invalid memory accesses. |
|
199 |
|
|
200 |
[Currently, the virtual CPU cannot retrieve the exact CPU state in some |
|
201 |
exceptions, although it could except for the @code{EFLAGS} register]. |
|
202 |
|
|
203 |
@section Linux system call translation |
|
204 |
|
|
205 |
QEMU includes a generic system call translator for Linux. It means that |
|
206 |
the parameters of the system calls can be converted to fix the |
|
207 |
endianness and 32/64 bit issues. The IOCTLs are converted with a generic |
|
208 |
type description system (see @file{ioctls.h} and @file{thunk.c}). |
|
209 |
|
|
210 |
@section Linux signals |
|
211 |
|
|
212 |
Normal and real-time signals are queued along with their information |
|
213 |
(@code{siginfo_t}) as it is done in the Linux kernel. Then an interrupt |
|
214 |
request is done to the virtual CPU. When it is interrupted, one queued |
|
215 |
signal is handled by generating a stack frame in the virtual CPU as the |
|
216 |
Linux kernel does. The @code{sigreturn()} system call is emulated to return |
|
217 |
from the virtual signal handler. |
|
218 |
|
|
219 |
Some signals (such as SIGALRM) directly come from the host. Other |
|
220 |
signals are synthetized from the virtual CPU exceptions such as SIGFPE |
|
221 |
when a division by zero is done (see @code{main.c:cpu_loop()}). |
|
222 |
|
|
223 |
The blocked signal mask is still handled by the host Linux kernel so |
|
224 |
that most signal system calls can be redirected directly to the host |
|
225 |
Linux kernel. Only the @code{sigaction()} and @code{sigreturn()} system |
|
226 |
calls need to be fully emulated (see @file{signal.c}). |
|
227 |
|
|
228 |
@section clone() system call and threads |
|
229 |
|
|
230 |
The Linux clone() system call is usually used to create a thread. QEMU |
|
231 |
uses the host clone() system call so that real host threads are created |
|
232 |
for each emulated thread. One virtual CPU instance is created for each |
|
233 |
thread. |
|
234 |
|
|
235 |
The virtual x86 CPU atomic operations are emulated with a global lock so |
|
236 |
that their semantic is preserved. |
|
237 |
|
|
238 |
@section Bibliography |
|
239 |
|
|
240 |
@table @asis |
|
241 |
|
|
242 |
@item [1] |
|
243 |
@url{http://citeseer.nj.nec.com/piumarta98optimizing.html}, Optimizing |
|
244 |
direct threaded code by selective inlining (1998) by Ian Piumarta, Fabio |
|
245 |
Riccardi. |
|
246 |
|
|
247 |
@item [2] |
|
248 |
@url{http://developer.kde.org/~sewardj/}, Valgrind, an open-source |
|
249 |
memory debugger for x86-GNU/Linux, by Julian Seward. |
|
250 |
|
|
251 |
@item [3] |
|
252 |
@url{http://bochs.sourceforge.net/}, the Bochs IA-32 Emulator Project, |
|
253 |
by Kevin Lawton et al. |
|
254 |
|
|
255 |
@item [4] |
|
256 |
@url{http://www.cs.rose-hulman.edu/~donaldlf/em86/index.html}, the EM86 |
|
257 |
x86 emulator on Alpha-Linux. |
|
258 |
|
|
259 |
@item [5] |
|
260 |
@url{http://www.usenix.org/publications/library/proceedings/usenix-nt97/full_papers/chernoff/chernoff.pdf}, |
|
261 |
DIGITAL FX!32: Running 32-Bit x86 Applications on Alpha NT, by Anton |
|
262 |
Chernoff and Ray Hookway. |
|
263 |
|
|
264 |
@end table |
|
265 |
|
|
266 |
@chapter Regression Tests |
|
267 |
|
|
268 |
In the directory @file{tests/}, various interesting x86 testing programs |
|
269 |
are available. There are used for regression testing. |
|
270 |
|
|
271 |
@section @file{hello} |
|
272 |
|
|
273 |
Very simple statically linked x86 program, just to test QEMU during a |
|
274 |
port to a new host CPU. |
|
275 |
|
|
276 |
@section @file{test-i386} |
|
277 |
|
|
278 |
This program executes most of the 16 bit and 32 bit x86 instructions and |
|
279 |
generates a text output. It can be compared with the output obtained with |
|
280 |
a real CPU or another emulator. The target @code{make test} runs this |
|
281 |
program and a @code{diff} on the generated output. |
|
282 |
|
|
283 |
The Linux system call @code{modify_ldt()} is used to create x86 selectors |
|
284 |
to test some 16 bit addressing and 32 bit with segmentation cases. |
|
285 |
|
|
286 |
@section @file{testsig} |
|
287 |
|
|
288 |
This program tests various signal cases, including SIGFPE, SIGSEGV and |
|
289 |
SIGILL. |
|
290 |
|
|
291 |
@section @file{testclone} |
|
292 |
|
|
293 |
Tests the @code{clone()} system call (basic test). |
|
294 |
|
|
295 |
@section @file{testthread} |
|
296 |
|
|
297 |
Tests the glibc threads (more complicated than @code{clone()} because signals |
|
298 |
are also used). |
|
299 |
|
|
300 |
@section @file{sha1} |
|
301 |
|
|
302 |
It is a simple benchmark. Care must be taken to interpret the results |
|
303 |
because it mostly tests the ability of the virtual CPU to optimize the |
|
304 |
@code{rol} x86 instruction and the condition code computations. |
|
305 |
|
Also available in: Unified diff