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1.1 root 1: TCG Interpreter (TCI) - Copyright (c) 2011 Stefan Weil.
2:
3: This file is released under the BSD license.
4:
5: 1) Introduction
6:
7: TCG (Tiny Code Generator) is a code generator which translates
8: code fragments ("basic blocks") from target code (any of the
9: targets supported by QEMU) to a code representation which
10: can be run on a host.
11:
12: QEMU can create native code for some hosts (arm, hppa, i386, ia64, ppc, ppc64,
13: s390, sparc, x86_64). For others, unofficial host support was written.
14:
15: By adding a code generator for a virtual machine and using an
16: interpreter for the generated bytecode, it is possible to
17: support (almost) any host.
18:
19: This is what TCI (Tiny Code Interpreter) does.
20:
21: 2) Implementation
22:
23: Like each TCG host frontend, TCI implements the code generator in
24: tcg-target.c, tcg-target.h. Both files are in directory tcg/tci.
25:
26: The additional file tcg/tci.c adds the interpreter.
27:
28: The bytecode consists of opcodes (same numeric values as those used by
29: TCG), command length and arguments of variable size and number.
30:
31: 3) Usage
32:
33: For hosts without native TCG, the interpreter TCI must be enabled by
34:
35: configure --enable-tcg-interpreter
36:
37: If configure is called without --enable-tcg-interpreter, it will
38: suggest using this option. Setting it automatically would need
39: additional code in configure which must be fixed when new native TCG
40: implementations are added.
41:
42: System emulation should work on any 32 or 64 bit host.
43: User mode emulation might work. Maybe a new linker script (*.ld)
44: is needed. Byte order might be wrong (on big endian hosts)
45: and need fixes in configure.
46:
47: For hosts with native TCG, the interpreter TCI can be enabled by
48:
49: configure --enable-tcg-interpreter
50:
51: The only difference from running QEMU with TCI to running without TCI
52: should be speed. Especially during development of TCI, it was very
53: useful to compare runs with and without TCI. Create /tmp/qemu.log by
54:
55: qemu-system-i386 -d in_asm,op_opt,cpu -singlestep
56:
57: once with interpreter and once without interpreter and compare the resulting
58: qemu.log files. This is also useful to see the effects of additional
59: registers or additional opcodes (it is easy to modify the virtual machine).
60: It can also be used to verify native TCGs.
61:
62: Hosts with native TCG can also enable TCI by claiming to be unsupported:
63:
64: configure --cpu=unknown --enable-tcg-interpreter
65:
66: configure then no longer uses the native linker script (*.ld) for
67: user mode emulation.
68:
69:
70: 4) Status
71:
72: TCI needs special implementation for 32 and 64 bit host, 32 and 64 bit target,
73: host and target with same or different endianness.
74:
75: | host (le) host (be)
76: | 32 64 32 64
77: ------------+------------------------------------------------------------
78: target (le) | s0, u0 s1, u1 s?, u? s?, u?
79: 32 bit |
80: |
81: target (le) | sc, uc s1, u1 s?, u? s?, u?
82: 64 bit |
83: |
84: target (be) | sc, u0 sc, uc s?, u? s?, u?
85: 32 bit |
86: |
87: target (be) | sc, uc sc, uc s?, u? s?, u?
88: 64 bit |
89: |
90:
91: System emulation
92: s? = untested
93: sc = compiles
94: s0 = bios works
95: s1 = grub works
96: s2 = Linux boots
97:
98: Linux user mode emulation
99: u? = untested
100: uc = compiles
101: u0 = static hello works
102: u1 = linux-user-test works
103:
104: 5) Todo list
105:
106: * TCI is not widely tested. It was written and tested on a x86_64 host
107: running i386 and x86_64 system emulation and Linux user mode.
108: A cross compiled QEMU for i386 host also works with the same basic tests.
109: A cross compiled QEMU for mipsel host works, too. It is terribly slow
110: because I run it in a mips malta emulation, so it is an interpreted
111: emulation in an emulation.
112: A cross compiled QEMU for arm host works (tested with pc bios).
113: A cross compiled QEMU for ppc host works at least partially:
114: i386-linux-user/qemu-i386 can run a simple hello-world program
115: (tested in a ppc emulation).
116:
117: * Some TCG opcodes are either missing in the code generator and/or
118: in the interpreter. These opcodes raise a runtime exception, so it is
119: possible to see where code must be added.
120:
121: * The pseudo code is not optimized and still ugly. For hosts with special
122: alignment requirements, it needs some fixes (maybe aligned bytecode
123: would also improve speed for hosts which support byte alignment).
124:
125: * A better disassembler for the pseudo code would be nice (a very primitive
126: disassembler is included in tcg-target.c).
127:
128: * It might be useful to have a runtime option which selects the native TCG
129: or TCI, so QEMU would have to include two TCGs. Today, selecting TCI
130: is a configure option, so you need two compilations of QEMU.
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