Frequently Asked Questions for Developers (Developer FAQ)

• Q: How does VirtualBox relate to QEMU? I have seen some source files that appear to be related to his project.

A: VirtualBox makes use of QEMU in two ways: first of all, some of our virtual hardware devices have their origin in the QEMU project. We have found them to be very useful and took them as a starting point. Secondly, we have incorporated the recompiler of QEMU as a fallback mechanism for situations where our Virtual Machine Manager (VMM) cannot correctly handle a certain situation. This holds true for guest code that executes in real mode (such as bootup) and some rare scenarios at runtime. QEMU saved us a lot of work and we are very grateful to be able to rely on it.

• Q: Does VirtualBox support CPU virtualization enhancements such as Intel VT-x (codename Vanderpool) and AMD SVM?

A: Yes, we provide full support for Intel VT-x and are working on adding support for AMD SVM. However, we do not make use of these features by default. You have to enable it either globally or on a per VM basis. The reason is very simple: our x86 virtualization is very sophisticated and in most cases it provides significantly better performance than when relying on VT-x. Virtualization products that rely on VT-x are usually much less sophisticated and tuned. With VT-x, a special CPU environment has to be entered in order to execute guest code and whenever activity of the VMM is required, this environment has to be left and then entered again. This is painfully slow and will take some years until additional benefits of VT-x and SVM (such as nested paging) may outweigh the performance penalty. VirtualBox will continue to support the latest CPU enhancements and already today, you benefit from VT-x when running some exotic guest operating systems such as OS/2 Warp. In general, with VT-x enabled much less virtualization code from VirtualBox has to be executed which can result in a more reliable system in case there are problems. So if you run into an issue, we recommend to compare the results to a VT-x enabled VM.