Understanding qemu-kvm

1
votes

On a remote machine, I have qemu-x86_64 installed. Upon trying to find the version of the same I'm presented with the following information.

$ qemu-x86_64 -version
qemu-x86_64 version 1.0 (qemu-kvm-1.0), Copyright (c) 2003-2008 Fabrice Bellard

I'm trying to understand what qemu-kvm is. We will not discuss whole system emulation but only qemu user level emulation.

QEMU supports 2 kinds of emulation : system and user level. In System level emulation the whole system is emulated, and you see that an OS can be booted up using the same. In User level emulation, I'm able to run binaries compiled for an architecture on another architecture. eg: I end up being able to run Linux MIPS binaries on an x86-64 machine.

The version information for qemu-x86_64 on my machine is as follows.

qemu-x86_64 version 2.2.0 (Debian 1:2.2+dfsg-5expubuntu9.2), Copyright (c) 2003-2008 Fabrice Bellard

I'm trying to understand what kind of a qemu-x86_64 is running on the remote machine. Where does kvm fit in? The remote machine is also a 64-bit machine.

When I run a 64-bit binary on the remote machine using its qemu-x86_64, there is no binary translation going on, instead qemu is using KVM to execute the instructions on actual hardware. If so, what part does qemu play? Does it handle privileged instructions? I'm trying to understand where exactly kvm comes into the picture.

1
linuxemulationqemukvm

1 Answers

2
votes

In essential binary-translation allows you to run instructions of another architecture (e.g., MIPS) on your physical machine. The target architecture is simulated. For example, the registers in the simulated MIPS machine are just some variables in the QEMU process.

It is true that QEMU can use binary-translation to simulate a x86_64 machine on your computer. However because it's simulating a same architecture, the instructions can actually be directly executed by the host machine without translation! QEMU employs some techniques which use the hardware supports from CPU and OS/software supports such as KVM/Xen. It's still simulation, or you can call it virtualization.