qemu-tech.texi 14 KB
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@node Implementation notes
@appendix Implementation notes

@menu
* CPU emulation::
* Translator Internals::
* QEMU compared to other emulators::
* Managed start up options::
* Bibliography::
@end menu

@node CPU emulation
@section CPU emulation

@menu
* x86::     x86 and x86-64 emulation
* ARM::     ARM emulation
* MIPS::    MIPS emulation
* PPC::     PowerPC emulation
* SPARC::   Sparc32 and Sparc64 emulation
* Xtensa::  Xtensa emulation
@end menu

@node x86
@subsection x86 and x86-64 emulation

QEMU x86 target features:

@itemize

@item The virtual x86 CPU supports 16 bit and 32 bit addressing with segmentation.
LDT/GDT and IDT are emulated. VM86 mode is also supported to run
DOSEMU. There is some support for MMX/3DNow!, SSE, SSE2, SSE3, SSSE3,
and SSE4 as well as x86-64 SVM.

@item Support of host page sizes bigger than 4KB in user mode emulation.

@item QEMU can emulate itself on x86.

@item An extensive Linux x86 CPU test program is included @file{tests/test-i386}.
It can be used to test other x86 virtual CPUs.

@end itemize

Current QEMU limitations:

@itemize

@item Limited x86-64 support.

@item IPC syscalls are missing.

@item The x86 segment limits and access rights are not tested at every
memory access (yet). Hopefully, very few OSes seem to rely on that for
normal use.

@end itemize

@node ARM
@subsection ARM emulation

@itemize

@item Full ARM 7 user emulation.

@item NWFPE FPU support included in user Linux emulation.

@item Can run most ARM Linux binaries.

@end itemize

@node MIPS
@subsection MIPS emulation

@itemize

@item The system emulation allows full MIPS32/MIPS64 Release 2 emulation,
including privileged instructions, FPU and MMU, in both little and big
endian modes.

@item The Linux userland emulation can run many 32 bit MIPS Linux binaries.

@end itemize

Current QEMU limitations:

@itemize

@item Self-modifying code is not always handled correctly.

@item 64 bit userland emulation is not implemented.

@item The system emulation is not complete enough to run real firmware.

@item The watchpoint debug facility is not implemented.

@end itemize

@node PPC
@subsection PowerPC emulation

@itemize

@item Full PowerPC 32 bit emulation, including privileged instructions,
FPU and MMU.

@item Can run most PowerPC Linux binaries.

@end itemize

@node SPARC
@subsection Sparc32 and Sparc64 emulation

@itemize

@item Full SPARC V8 emulation, including privileged
instructions, FPU and MMU. SPARC V9 emulation includes most privileged
and VIS instructions, FPU and I/D MMU. Alignment is fully enforced.

@item Can run most 32-bit SPARC Linux binaries, SPARC32PLUS Linux binaries and
some 64-bit SPARC Linux binaries.

@end itemize

Current QEMU limitations:

@itemize

@item IPC syscalls are missing.

@item Floating point exception support is buggy.

@item Atomic instructions are not correctly implemented.

@item There are still some problems with Sparc64 emulators.

@end itemize

@node Xtensa
@subsection Xtensa emulation

@itemize

@item Core Xtensa ISA emulation, including most options: code density,
loop, extended L32R, 16- and 32-bit multiplication, 32-bit division,
MAC16, miscellaneous operations, boolean, FP coprocessor, coprocessor
context, debug, multiprocessor synchronization,
conditional store, exceptions, relocatable vectors, unaligned exception,
interrupts (including high priority and timer), hardware alignment,
region protection, region translation, MMU, windowed registers, thread
pointer, processor ID.

@item Not implemented options: data/instruction cache (including cache
prefetch and locking), XLMI, processor interface. Also options not
covered by the core ISA (e.g. FLIX, wide branches) are not implemented.

@item Can run most Xtensa Linux binaries.

@item New core configuration that requires no additional instructions
may be created from overlay with minimal amount of hand-written code.

@end itemize

@node Translator Internals
@section Translator Internals

QEMU is a dynamic translator. When it first encounters a piece of code,
it converts it to the host instruction set. Usually dynamic translators
are very complicated and highly CPU dependent. QEMU uses some tricks
which make it relatively easily portable and simple while achieving good
performances.

QEMU's dynamic translation backend is called TCG, for "Tiny Code
Generator". For more information, please take a look at @code{tcg/README}.

Some notable features of QEMU's dynamic translator are:

@table @strong

@item CPU state optimisations:
The target CPUs have many internal states which change the way it
evaluates instructions. In order to achieve a good speed, the
translation phase considers that some state information of the virtual
CPU cannot change in it. The state is recorded in the Translation
Block (TB). If the state changes (e.g. privilege level), a new TB will
be generated and the previous TB won't be used anymore until the state
matches the state recorded in the previous TB. The same idea can be applied
to other aspects of the CPU state.  For example, on x86, if the SS,
DS and ES segments have a zero base, then the translator does not even
generate an addition for the segment base.

@item Direct block chaining:
After each translated basic block is executed, QEMU uses the simulated
Program Counter (PC) and other cpu state information (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 host
architectures (such as x86 or PowerPC), the @code{JUMP} opcode is
directly patched so that the block chaining has no overhead.

@item 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.

User-mode emulation marks a host page as write-protected (if it is
not already read-only) every time translated code is generated for a
basic block.  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.  For system emulation, write
protection is achieved through the software MMU.

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.

On RISC targets, correctly written software uses memory barriers and
cache flushes, so some of the protection above would not be
necessary. However, QEMU still requires that the generated code always
matches the target instructions in memory in order to handle
exceptions correctly.

@item 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.  QEMU keeps a map from host program counter to
target program counter, and looks up where the exception happened
based on the host program counter at the exception point.

On some targets, some bits of the virtual CPU's state are not flushed to the
memory until the end of the translation block.  This is done for internal
emulation state that is rarely accessed directly by the program and/or changes
very often throughout the execution of a translation block---this includes
condition codes on x86, delay slots on SPARC, conditional execution on
ARM, and so on.  This state is stored for each target instruction, and
looked up on exceptions.

@item MMU emulation:
For system emulation QEMU uses a software MMU. In that mode, the MMU
virtual to physical address translation is done at every memory
access.

QEMU uses an address translation cache (TLB) to speed up the translation.
In order to avoid flushing the translated code each time the MMU
mappings change, all caches in QEMU are physically indexed.  This
means that each basic block is indexed with its physical address.

In order to avoid invalidating the basic block chain when MMU mappings
change, chaining is only performed when the destination of the jump
shares a page with the basic block that is performing the jump.

The MMU can also distinguish RAM and ROM memory areas from MMIO memory
areas.  Access is faster for RAM and ROM because the translation cache also
hosts the offset between guest address and host memory.  Accessing MMIO
memory areas instead calls out to C code for device emulation.
Finally, the MMU helps tracking dirty pages and pages pointed to by
translation blocks.
@end table

@node QEMU compared to other emulators
@section QEMU compared to other emulators

Like bochs [1], QEMU emulates an x86 CPU. But QEMU is much faster than
bochs as it uses dynamic compilation. Bochs is closely tied to x86 PC
emulation while QEMU can emulate several processors.

Like Valgrind [2], QEMU does user space emulation and dynamic
translation. Valgrind is mainly a memory debugger while QEMU has no
support for it (QEMU could be used to detect out of bound memory
accesses as Valgrind, but it has no support to track uninitialised data
as Valgrind does). The Valgrind dynamic translator generates better code
than QEMU (in particular it does register allocation) but it is closely
tied to an x86 host and target and has no support for precise exceptions
and system emulation.

EM86 [3] is the closest project to user space QEMU (and QEMU still uses
some of its code, in particular the ELF file loader). EM86 was limited
to an alpha host and used a proprietary and slow interpreter (the
interpreter part of the FX!32 Digital Win32 code translator [4]).

TWIN from Willows Software was a Windows API emulator like Wine. It is less
accurate than Wine but includes a protected mode x86 interpreter to launch
x86 Windows executables. Such an approach has greater potential because most
of the Windows API is executed natively but it is far more difficult to
develop because all the data structures and function parameters exchanged
between the API and the x86 code must be converted.

User mode Linux [5] was the only solution before QEMU to launch a
Linux kernel as a process while not needing any host kernel
patches. However, user mode Linux requires heavy kernel patches while
QEMU accepts unpatched Linux kernels. The price to pay is that QEMU is
slower.

The Plex86 [6] PC virtualizer is done in the same spirit as the now
obsolete qemu-fast system emulator. It requires a patched Linux kernel
to work (you cannot launch the same kernel on your PC), but the
patches are really small. As it is a PC virtualizer (no emulation is
done except for some privileged instructions), it has the potential of
being faster than QEMU. The downside is that a complicated (and
potentially unsafe) host kernel patch is needed.

The commercial PC Virtualizers (VMWare [7], VirtualPC [8]) are faster
than QEMU (without virtualization), but they all need specific, proprietary
and potentially unsafe host drivers. Moreover, they are unable to
provide cycle exact simulation as an emulator can.

VirtualBox [9], Xen [10] and KVM [11] are based on QEMU. QEMU-SystemC
[12] uses QEMU to simulate a system where some hardware devices are
developed in SystemC.

@node Managed start up options
@section Managed start up options

In system mode emulation, it's possible to create a VM in a paused state using
the -S command line option. In this state the machine is completely initialized
according to command line options and ready to execute VM code but VCPU threads
are not executing any code. The VM state in this paused state depends on the way
QEMU was started. It could be in:
@table @asis
@item initial state (after reset/power on state)
@item with direct kernel loading, the initial state could be amended to execute
code loaded by QEMU in the VM's RAM and with incoming migration
@item with incoming migration, initial state will by amended with the migrated
machine state after migration completes.
@end table

This paused state is typically used by users to query machine state and/or
additionally configure the machine (by hotplugging devices) in runtime before
allowing VM code to run.

However, at the -S pause point, it's impossible to configure options that affect
initial VM creation (like: -smp/-m/-numa ...) or cold plug devices. The
experimental --preconfig command line option  allows pausing QEMU
before the initial VM creation, in a ``preconfig'' state, where additional
queries and configuration can be performed via QMP before moving on to
the resulting configuration startup. In the preconfig state, QEMU only allows
a limited set of commands over the QMP monitor, where the commands do not
depend on an initialized machine, including but not limited to:
@table @asis
@item qmp_capabilities
@item query-qmp-schema
@item query-commands
@item query-status
@item x-exit-preconfig
@end table

@node Bibliography
@section Bibliography

@table @asis

@item [1]
@url{http://bochs.sourceforge.net/}, the Bochs IA-32 Emulator Project,
by Kevin Lawton et al.

@item [2]
@url{http://www.valgrind.org/}, Valgrind, an open-source memory debugger
for GNU/Linux.

@item [3]
@url{http://ftp.dreamtime.org/pub/linux/Linux-Alpha/em86/v0.2/docs/em86.html},
the EM86 x86 emulator on Alpha-Linux.

@item [4]
@url{http://www.usenix.org/publications/library/proceedings/usenix-nt97/@/full_papers/chernoff/chernoff.pdf},
DIGITAL FX!32: Running 32-Bit x86 Applications on Alpha NT, by Anton
Chernoff and Ray Hookway.

@item [5]
@url{http://user-mode-linux.sourceforge.net/},
The User-mode Linux Kernel.

@item [6]
@url{http://www.plex86.org/},
The new Plex86 project.

@item [7]
@url{http://www.vmware.com/},
The VMWare PC virtualizer.

@item [8]
@url{https://www.microsoft.com/download/details.aspx?id=3702},
The VirtualPC PC virtualizer.

@item [9]
@url{http://virtualbox.org/},
The VirtualBox PC virtualizer.

@item [10]
@url{http://www.xen.org/},
The Xen hypervisor.

@item [11]
@url{http://www.linux-kvm.org/},
Kernel Based Virtual Machine (KVM).

@item [12]
@url{http://www.greensocs.com/projects/QEMUSystemC},
QEMU-SystemC, a hardware co-simulator.

@end table