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Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
131 lines
4.6 KiB
Plaintext
131 lines
4.6 KiB
Plaintext
TCG Interpreter (TCI) - Copyright (c) 2011 Stefan Weil.
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This file is released under the BSD license.
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1) Introduction
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TCG (Tiny Code Generator) is a code generator which translates
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code fragments ("basic blocks") from target code (any of the
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targets supported by QEMU) to a code representation which
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can be run on a host.
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QEMU can create native code for some hosts (arm, i386, ia64, ppc, ppc64,
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s390, sparc, x86_64). For others, unofficial host support was written.
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By adding a code generator for a virtual machine and using an
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interpreter for the generated bytecode, it is possible to
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support (almost) any host.
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This is what TCI (Tiny Code Interpreter) does.
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2) Implementation
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Like each TCG host frontend, TCI implements the code generator in
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tcg-target.c.inc, tcg-target.h. Both files are in directory tcg/tci.
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The additional file tcg/tci.c adds the interpreter.
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The bytecode consists of opcodes (same numeric values as those used by
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TCG), command length and arguments of variable size and number.
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3) Usage
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For hosts without native TCG, the interpreter TCI must be enabled by
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configure --enable-tcg-interpreter
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If configure is called without --enable-tcg-interpreter, it will
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suggest using this option. Setting it automatically would need
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additional code in configure which must be fixed when new native TCG
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implementations are added.
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System emulation should work on any 32 or 64 bit host.
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User mode emulation might work. Maybe a new linker script (*.ld)
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is needed. Byte order might be wrong (on big endian hosts)
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and need fixes in configure.
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For hosts with native TCG, the interpreter TCI can be enabled by
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configure --enable-tcg-interpreter
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The only difference from running QEMU with TCI to running without TCI
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should be speed. Especially during development of TCI, it was very
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useful to compare runs with and without TCI. Create /tmp/qemu.log by
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qemu-system-i386 -d in_asm,op_opt,cpu -D /tmp/qemu.log -singlestep
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once with interpreter and once without interpreter and compare the resulting
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qemu.log files. This is also useful to see the effects of additional
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registers or additional opcodes (it is easy to modify the virtual machine).
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It can also be used to verify native TCGs.
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Hosts with native TCG can also enable TCI by claiming to be unsupported:
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configure --cpu=unknown --enable-tcg-interpreter
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configure then no longer uses the native linker script (*.ld) for
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user mode emulation.
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4) Status
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TCI needs special implementation for 32 and 64 bit host, 32 and 64 bit target,
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host and target with same or different endianness.
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| host (le) host (be)
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| 32 64 32 64
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------------+------------------------------------------------------------
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target (le) | s0, u0 s1, u1 s?, u? s?, u?
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32 bit |
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target (le) | sc, uc s1, u1 s?, u? s?, u?
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64 bit |
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target (be) | sc, u0 sc, uc s?, u? s?, u?
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32 bit |
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target (be) | sc, uc sc, uc s?, u? s?, u?
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64 bit |
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System emulation
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s? = untested
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sc = compiles
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s0 = bios works
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s1 = grub works
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s2 = Linux boots
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Linux user mode emulation
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u? = untested
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uc = compiles
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u0 = static hello works
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u1 = linux-user-test works
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5) Todo list
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* TCI is not widely tested. It was written and tested on a x86_64 host
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running i386 and x86_64 system emulation and Linux user mode.
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A cross compiled QEMU for i386 host also works with the same basic tests.
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A cross compiled QEMU for mipsel host works, too. It is terribly slow
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because I run it in a mips malta emulation, so it is an interpreted
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emulation in an emulation.
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A cross compiled QEMU for arm host works (tested with pc bios).
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A cross compiled QEMU for ppc host works at least partially:
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i386-linux-user/qemu-i386 can run a simple hello-world program
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(tested in a ppc emulation).
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* Some TCG opcodes are either missing in the code generator and/or
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in the interpreter. These opcodes raise a runtime exception, so it is
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possible to see where code must be added.
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* The pseudo code is not optimized and still ugly. For hosts with special
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alignment requirements, it needs some fixes (maybe aligned bytecode
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would also improve speed for hosts which support byte alignment).
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* A better disassembler for the pseudo code would be nice (a very primitive
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disassembler is included in tcg-target.c.inc).
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* It might be useful to have a runtime option which selects the native TCG
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or TCI, so QEMU would have to include two TCGs. Today, selecting TCI
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is a configure option, so you need two compilations of QEMU.
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