1TCG Interpreter (TCI) - Copyright (c) 2011 Stefan Weil.
2
3This file is released under the BSD license.
4
51) Introduction
6
7TCG (Tiny Code Generator) is a code generator which translates
8code fragments ("basic blocks") from target code (any of the
9targets supported by QEMU) to a code representation which
10can be run on a host.
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12QEMU can create native code for some hosts (arm, i386, ia64, ppc, ppc64,
13s390, sparc, x86_64). For others, unofficial host support was written.
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15By adding a code generator for a virtual machine and using an
16interpreter for the generated bytecode, it is possible to
17support (almost) any host.
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19This is what TCI (Tiny Code Interpreter) does.
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212) Implementation
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23Like each TCG host frontend, TCI implements the code generator in
24tcg-target.c.inc, tcg-target.h. Both files are in directory tcg/tci.
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26The additional file tcg/tci.c adds the interpreter and disassembler.
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28The bytecode consists of opcodes (with only a few exceptions, with
29the same same numeric values and semantics as used by TCG), and up
30to six arguments packed into a 32-bit integer. See comments in tci.c
31for details on the encoding.
32
333) Usage
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35For hosts without native TCG, the interpreter TCI must be enabled by
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37 configure --enable-tcg-interpreter
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39If configure is called without --enable-tcg-interpreter, it will
40suggest using this option. Setting it automatically would need
41additional code in configure which must be fixed when new native TCG
42implementations are added.
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44For hosts with native TCG, the interpreter TCI can be enabled by
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46 configure --enable-tcg-interpreter
47
48The only difference from running QEMU with TCI to running without TCI
49should be speed. Especially during development of TCI, it was very
50useful to compare runs with and without TCI. Create /tmp/qemu.log by
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52 qemu-system-i386 -d in_asm,op_opt,cpu -D /tmp/qemu.log -accel tcg,one-insn-per-tb=on
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54once with interpreter and once without interpreter and compare the resulting
55qemu.log files. This is also useful to see the effects of additional
56registers or additional opcodes (it is easy to modify the virtual machine).
57It can also be used to verify native TCGs.
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59Hosts with native TCG can also enable TCI by claiming to be unsupported:
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61 configure --cpu=unknown --enable-tcg-interpreter
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63configure then no longer uses the native linker script (*.ld) for
64user mode emulation.
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66
674) Status
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69TCI needs special implementation for 32 and 64 bit host, 32 and 64 bit target,
70host and target with same or different endianness.
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72 | host (le) host (be)
73 | 32 64 32 64
74------------+------------------------------------------------------------
75target (le) | s0, u0 s1, u1 s?, u? s?, u?
7632 bit |
77 |
78target (le) | sc, uc s1, u1 s?, u? s?, u?
7964 bit |
80 |
81target (be) | sc, u0 sc, uc s?, u? s?, u?
8232 bit |
83 |
84target (be) | sc, uc sc, uc s?, u? s?, u?
8564 bit |
86 |
87
88System emulation
89s? = untested
90sc = compiles
91s0 = bios works
92s1 = grub works
93s2 = Linux boots
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95Linux user mode emulation
96u? = untested
97uc = compiles
98u0 = static hello works
99u1 = linux-user-test works
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1015) Todo list
102
103* TCI is not widely tested. It was written and tested on a x86_64 host
104 running i386 and x86_64 system emulation and Linux user mode.
105 A cross compiled QEMU for i386 host also works with the same basic tests.
106 A cross compiled QEMU for mipsel host works, too. It is terribly slow
107 because I run it in a mips malta emulation, so it is an interpreted
108 emulation in an emulation.
109 A cross compiled QEMU for arm host works (tested with pc bios).
110 A cross compiled QEMU for ppc host works at least partially:
111 i386-linux-user/qemu-i386 can run a simple hello-world program
112 (tested in a ppc emulation).
113
114* Some TCG opcodes are either missing in the code generator and/or
115 in the interpreter. These opcodes raise a runtime exception, so it is
116 possible to see where code must be added.
117
118* It might be useful to have a runtime option which selects the native TCG
119 or TCI, so QEMU would have to include two TCGs. Today, selecting TCI
120 is a configure option, so you need two compilations of QEMU.
121