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Real to Virtual, and Back Again using none toincoporate none for web,windows data matrix One use for X none none en is to run a legacy operating system inside a newer one. Although Xen, in fact, runs underneath both, the illusion that one OS is running as the guest of another is easy to maintain by exporting the user interface components from one guest to another. Legacy operating systems typically have a large number of custom con gu253.

QR Code Module Size and Area 14. Future Directions ration option none for none s set, and so reinstalling them in a virtualized environment is not always possible. In these instances, it is highly bene cial to be able to move them directly from their current location into a virtual domain. In some cases, it might be useful to be able to dual boot the legacy operating system, and either run it in native mode or in a virtual environment.

A lot of work has been done with Xen to allow this. Windows, for example, can be dual-booted with XenoLinux, and either run natively or in an HVM guest. Because Xen does not currently support 3D acceleration of Windows guests, it can be used in a virtual environment for most legacy applications and then run natively when 3D support is required.

This facility can be used in conjunction with HVM-assisted virtualization as well, to allow an operating system to be installed as an HVM guest and then have the kernel replaced with a paravirtualization-aware version. This allows an operating system to be installed from original media without the need to reboot the computer without Xen for the install process. The biggest problem with this approach is that installers boot in real mode, and often do some very messy things at the start.

This is especially true of systems that boot from CD. On AMD systems, which permit virtualization of real mode guests, this is less of a problem; but on Intel systems, it requires emulation of much of real mode. Currently, the emulation code in Xen is undergoing a lot of work to make this process relatively painless.

. Emulation and Virtualization Virtualizatio none none n can be seen as a special case of emulation, where the emulation function for the majority of operations is the identity function. Because it is already possible to migrate running machines from one Xen host to another, this leads to the question of whether it is possible to migrate one to a full emulator instead. Conceptually, this is not di cult.

During migration, the hypervisor is aware of the entire virtual machine state register and memory contents as well as device states. This information can be used to set the emulator state and allow things to continue. Running a virtual machine in emulation provides much ner-grained control over operations.

An emulator can easily be run one instruction at a time, and in some cases even run backward. Input and output are strictly controlled, and this makes it an ideal platform for debugging. Debugging is not limited to the software.

If a virtual machine is experiencing problems at a particular point, it is possible to compare the results of the emulator to those of a real machine and see if there are discrepancies. Because it is often easier to verify the implementation of an instruction, or set of instructions, in an emulator than on a physical machine, this can be useful in debugging the hardware. 14.3. Porting E orts as well. Of c none none ourse, the bug may be in the emulator; again the capability to run a machine from the same state on both real and emulated hardware can help debugging here. Beyond debugging, migration to emulated environments opens the door to cross-platform migration of virtual machines.

A virtual machine running on a slow ARM chip in a mobile device could be migrated to a fast desktop or server machine. Even allowing for the overhead of emulation, this is likely to give a speed boost. If the guest kernel is aware of the migration, it can even take advantage of it directly.

Code running in abstract machines such as the JVM or .NET CLR could have the JID cache invalidated and be recompiled from bytecode for the new native platform. Currently, virtual to emulated (V2E ) and emulated to virtualized (E2V ) migrations using QEMU are still experimental.

They are likely to become a signi cant feature of Xen in the future, however..
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