QEMU compared to other emulatorsLike 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.