Hardware assisted virtualization

Intel and AMD realized that full virtualization and paravirtualization are the major challenges of virtualization on the x86 architecture (as the scope of this book is limited to x86 architecture, we will mainly discuss the evolution of this architecture here) due to the performance overhead and complexity in designing and maintaining the solution. Intel and AMD independently created new processor extensions of the x86 architecture, called Intel VT-x and AMD-V respectively. On the Itanium architecture, hardware-assisted virtualization is known as VT-i. Hardware assisted virtualization is a platform virtualization method designed to efficiently use full virtualization with the hardware capabilities. Various vendors call this technology by different names, including accelerated virtualization, hardware virtual machine, and native virtualization.

For better support of for virtualization, Intel and AMD introduced Virtualization Technology (VT) and Secure Virtual Machine (SVM), respectively, as extensions of the IA-32 instruction set. These extensions allow the VMM/hypervisor to run a guest OS that expects to run in kernel mode, in lower privileged rings. Hardware assisted virtualization not only proposes new instructions, but also introduces a new privileged access level, called ring -1, where the hypervisor/VMM can run. Hence, guest virtual machines can run in ring 0. With hardware-assisted virtualization, the operating system has direct access to resources without any emulation or OS modification. The hypervisor or VMM can now run at the newly introduced privilege level, Ring -1, with the guest operating systems running on Ring 0. Also, with hardware assisted virtualization, the VMM/hypervisor is relaxed and needs to perform less work compared to the other techniques mentioned, which reduces the performance overhead.

In simple terms, this virtualization-aware hardware provides the support to build the VMM and also ensures the isolation of a guest operating system. This helps to achieve better performance and avoid the complexity of designing a virtualization solution. Modern virtualization techniques make use of this feature to provide virtualization. One example is KVM, which we are going to discuss in detail in the scope of this book.