QEMU Interview Preparation Guide

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The place where you can find tuntap network mode configuration settings is /dev/tap0 or in the fle /dev/net/tap0. The tuntap network mode can be seen by using the command ifconfig -a. For tuntap mode to exist QEMU should be running and when it shuts down then the devices which are attached to it automatically gets shut down. During shut down it removes the tuntap devices as well.

QEMU is in the process of creating a kernel based virtual machine that uses modified QEMU to provide the extensions for different technologies. The two most processors that are used are Intel and AMD-V processors for which QEMU is creating this extensions. VT (virtualization technology) is disabled by BIOS by default but it can be enabled from BIOS menu to turn on all the functionalities of QEMU.

QEMU uses a tap device only the tun0 name which is being given allows the user to connect in the networking. The kernel of the system create a tap device named as tun0 and a tap device with name tap0. It just uses the name to differentiate its own configuration but it uses tap device only for networking methods. It uses Ethernet frames for the configuration to be stored.

To setup tuntap networking mode it requires the implication of tap networking device on the host system. It provides good performance overall and can be configured to create virtual network topology. At the same time, it requires high end configuration of the network topology in host that tends to differ from one operating system to another operating system. The command that allows it to run tap networking mode is:
-netdev tap

To change the network configuration for the user it requires

-netdev type=user.

The device option in the command is used to plug in the particular network device like

-netdeve type=user, id=mynet0 -device e2000, netdev= mynet0.

Qemu is a virtualization platform application that works as an emulator that emulates an existing system to run more than one operating system together at the same time. It has its own configuration and it is easy to install and use. It is used to emulate many subsystems like hardware and softwares. It emulates the processors as well like multiprocessing systems.

Qemu has two modes of operations and they are as follows:
• User mode emulation: this allow the process execution from one CPU to another CPU. It performs dynamic translation of instructions for host CPU.
• System mode emulation: this allow the emulation of the complete system that includes hardware as well as peripheral related components.

Dynamic translator translates the instructions that is being given by the guest system to the host system language. It translates the instructions at runtime to provide the emulation between the host CPU and guest CPU. To translate the instructions it maps the instructions that are passed from one CPU to another CPU. In this case there remains always the possibility that the behavior or additional instructions might be required by the architecture for translations.

To achieve high performance and using QEMU emulation environment there is a requirement to use QEMU accelerator that allows the execution of emulated codes and the libraries attached to it directly on the host CPU using the kernel module. It accelerates the translation of instructions from one CPU to another CPU without delaying the output to the user.

Dynamic translation can be achieved by first converting the target into micro-operations. Micro-operations in this case are in the form of C code that gets compiled into objects. A core translator is required that maps the target instructions into micro-operations instructions. It is an efficient and portable way to achieve translation using Qemu. It removes the overhead by caching the translated code. It supports self modification by making the translated block invalidate in cache.

To connect VLANs to tap devices the requirement is of vlan that should be available in the host operating system. Any frame that appears on this device can be attached using the QEMU process. This process request is received by another interfaces on VLAN and then frames are received by device. The command that is used to configure the networking mode:

qemu -net nic -net tap,ifname=qtap0 …

QEMU needs tuntap device driver that allow the user-space application to obtain different file discriptors that are connected to a network device. The tap devices can be seen by going to the directory file /dev/net/tun or else calling the function TUNSETIFF ioctl() that is allowed only to the user who are privileged.