Have you ever thought about how a single physical server in a cloud data center can possibly run dozens of all-independent virtual machines (VMs)? One can be a windows server with a site, another a Linux machine with a database and all can be running fine alongside each other without any interference with the other. This is not just a smart piece of software, it is a technological advancement and it needs some help in the form of the hardware that the software is dependent on.
This is the area of hardware-assisted virtualization, which is a breakthrough in nature and has made the revolution in cloud computing possible. That is why the services like Netflix, Spotify, and online banking can be very reliable, scaled and cost-effective.
In case you have ever been baffled by expressions like full virtualization, as opposed to paravirtualization, or what a hypervisor is and what is its role, you are not alone. All these concepts will be made clear in this guide. We are going to discuss what hardware virtualization is, how it works, why it can be considered such a great development, and why it may be relied upon when it comes to the most important applications in your business.
Understanding The Basic Problem
The hardware solution cannot be fully explored before we find out about the software problem. A hypervisor, or Virtual machine manager (VMM) is a software that forms the core of any virtualization infrastructure.
The work of the hypervisor is to develop and administer virtual machines. A VM is a complete computer with its own operating system (also known as the guest OS), software and virtual hardware, all running on a single physical system (the host).
The fundamental problem is with the CPU. Contemporary processors are constructed with the aim of being dominated by a special operating system. Some of the most important instructions, the ones that directly access the hardware, can only be run in a special mode of the operating system, what is commonly termed as the Ring 0.
So, what happens when you try to run a guest OS inside a VM?
- The hypervisor must run in Ring 0 to control the real hardware.
- The guest OS thinks it’s in Ring 0, but it’s actually trapped inside a VM.
- When the guest OS tries to execute a privileged instruction, it would normally take control of the entire CPU, crashing the physical server and all other VMs on it.
- This was the great challenge of early virtualization. Let’s see how engineers tried to solve it with software before hardware came to the rescue.
The Software Workarounds: Full and Paravirtualization
To handle this privilege problem, two primary software-based techniques were developed.
Full Virtualization
What it is: Full virtualization aims at executing the guest operating system completely unmodified. The hypervisor builds an exact and copious emulation of the hardware. In this case, the guest OS does not realize anything; it takes over the ownership of a real server as its own.
Mechanism: Hypervisor used a method known as Binary Translation. It was capable of scanning the code of the guest OS whilst running. Every time a privileged instruction with the potential to induce a fault was found, the hypervisor quietly redirected it and replaced it with a safe, “translated” version. The resulting virtual machine was fully operating, but the constant need of interception and translation added a significant load of performance.
Examples: The first versions of the VMware ESX server and Microsoft Virtual Server were the first to use this method. They could run unmodified standard, off-the-shelf Windows or Linux distributions.
Paravirtualization
What it is: Paravirtualization (a Greek word para meaning alongside) uses a more cooperative approach. It does not attempt to mislead the guest OS but admits that execution is in a virtualised environment.
- Mechanism: The operating system kernel of the guest is modified. All problematic privileged instructions are substituted by explicit calls of the hypervisor, called hypercalls. Simply put, the guest OS, in a polite manner, asks the hypervisor to do memory-management on its behalf.
- The Advantages and Disadvantages: This method is much more effective than binary translation because there is no need to intercept it with high cost. Its main disadvantage, though, is the fact that it requires a specially modified operating system; one cannot use a regular Windows ISO. As such, paravirtualization had a positive niche in the open-source Linux community (i.e., with Xen hypervisor) but became less feasible with proprietary or legacy operating systems.
Both methods worked, but they were complex and inefficient. The industry needed a cleaner, more fundamental solution. And that solution came from the CPU manufacturers themselves.
The Game Changer: Hardware-Assisted Virtualization
Around 2005-2006, both Intel and AMD introduced new CPU extensions to their processor architectures—Intel VT-x and AMD-V. This was hardware-assisted virtualization.
In simple terms, they added new CPU instruction sets and a new execution mode specifically designed for virtualization.
How Hardware Virtualization Works: A Simple Analogy
Consider the CPU to be a secure office building.
- Without Hardware Assist: The hypervisor (security guard) has to constantly follow the guest OS (employee) around, checking every document they try to file and often rewriting them. It works, but it’s slow and labor-intensive (high overhead).
- With Hardware Assist: The building architect (Intel/AMD) adds a new, secure filing room just for the employee. The employee can go into this room and file their own documents directly, but the room is designed so that they can’t access anyone else’s files or damage the building. The security guard (hypervisor) is still in charge, but doesn’t need to micromanage anymore.
Technically, what happens?
The CPU now has two distinct “modes”:
- Root Mode: This is for the hypervisor. It’s the true “Ring 0,” with full control over the hardware.
- Non-Root Mode: This is for the guest operating systems. They can run in their own privileged state, believing they are in control.
When a guest OS in Non-Root Mode tries to execute a sensitive instruction, the CPU itself automatically traps it and immediately hands control back to the hypervisor in Root Mode. This is called a VM Exit. The hypervisor handles the request, and then executes a VM Entry to return control to the guest.
This hardware-based “trap-and-emulate” model is incredibly efficient. It removes the need for slow binary translation and the need to modify the guest OS for paravirtualization. The hypervisor’s job becomes simpler, safer, and much faster.
Types of Hardware Virtualization
Even though CPU virtualization is the center of interest, other important elements will be involved in hardware-assisted concept:
- CPU Virtualization: This is the fundamental technology, and it is made possible by Intel VT-x and AMD-V as mentioned above.
- Memory Virtualization: This allows the hypervisor to manage memory using an efficient manner to each virtual machine. Intel EPT (Extended Page Tables) and AMD RVI (Rapid Virtualization Indexing) technologies allow the processor to support multiple VMs in making memory-address translations, thus significantly decreasing overhead.
- I/O Virtualization: This speeds up the network and storage operations. Some such technologies as Intel VT-d and AMD-Vi enable virtual machines to gain direct and secure access to physical I/O devices such as network cards by bypassing the hypervisor to transfer data to and from the hypervisor, thus reducing latency.
The New Role of the Hypervisor in Hardware-Assisted Environment
In hardware aided virtualization, the role of the hypervisor has become significantly changed. It is no longer a resource-consuming translator who finds himself incessantly between the guest operating system and central processing unit to mediate the communication. Rather, it has turned out to be a very effective planner and organizer of resources.
Take the example of a translator who needs to translate every sentence on the fly and that of a manager who has trained his staff to work in a standardized and effective reporting system. The burdens of the latter are much less. This has resulted in the hypervisor changing their role so as to be macro-managing rather than focusing on micromanaging with the aim of focusing on:
- Resource Allocation: This is used to determine how much CPU time, memory, and I/O bandwidth is allocated to each virtual machine.
- Scheduling: Determining what threads of each virtual machine are allowed to make use of the physical CPU cores, and how long.
- Lifecycle Management: Virtual machines Lifecycle Management deals with starting, stopping, and migrating of virtual machines.
- Virtual Devices Provision: The ability to have virtual equivalents of common hardware devices like network adapters or disk controller units, which do not require any custom drivers, which seem to be recognized by any guest operating system, and which that operating system can use.
This change enabled hypervisors like KVM (Kernel Based Virtual Machine). KVM is not a program, but it is a module which turns the Linux kernel itself into a hypervisor. With the Linux kernel already handling hardware, and the CPU handling the most challenging of virtualization tasks, KVM is very light and efficient to a point of being the de facto standard among many cloud providers today.
VMs to Containers: The Next Evolutionary Step
VMs became efficient with the help of hardware-supported virtualization, yet another technology has appeared: containerization, spearheaded by such tools as Docker and Kubernetes.
So how do they relate?
Whereas a VM virtualizes the whole hardware stack and creates a complete, independent operating system, a container virtualizes an operating system. Containers on a single host have the same host operating-system kernel. They are packaged, standalone units that consist of just the application and the library dependencies particular to that application.
Here the virtualization supported by hardware comes in surprising and continuous. Containers are based on the same principle of isolation to work safely. The software mechanisms that facilitate containers to work are features in the Linux kernel, including namespaces, which give isolated access to the system, and cgroups, which control resources. In addition, to provide full security in a multi-tenant cloud setup, vendors often operate containers within lightweight VMs. This approach offers a two-tier isolation, where the hardware ensures that the VM boundary does not get compromised.
Glossary: cgroups, or control groups, are a Linux kernel feature that limits, accounts for, and isolates the resource usage of a collection of processes
Actually, technologies like Kata Containers and gVisor develop this strategy. They operate like and look like regular containers, but, at the hood, each container is running in its own small, high-performance virtual machine with its own kernel, thus, using hardware virtualization to provide bulletproof security without affecting the developer experience of containers.
Real-World Impact: Why This All Matters to You
You might find that this discussion is interesting, but are left wondering what is the implication of this to your enterprise or projects. The implication extends and is physical.
The Real Birth of the Public Cloud
Before the hardware-assisted virtualization, the so-called multi-tenant cloud model, in which one physical server is used by the virtual devices of an innumerable number of different customers, was either too slow or too insecure to be considered workable. This technology provided the performance and high-level isolation that companies need to put their sensitive data and valuable applications on the cloud and make services like Amazon EC2 and Google Compute Engine a feasible and effective option.
Better Disaster Recovery and Business Continuity
Hardware-assisted live migration, or moving an operating virtual machine on one physical machine to a different physical machine with no downtime, is directly beneficial. It allows the cloud providers to carry out proactive maintenance, hardware upgrade as well as load balancing seamlessly.
Software Developer Agility
This technology is very beneficial to the software developers. It allows one to run local machines with tools like VirtualBox or VMware to create isolated and reproducible development and testing environments. One can test an application on Windows, Linux and macOS in the same laptop and does not require an entire rack of physical servers. This enhances the speed of software development and software quality.
Cost Reductions and Sustainability
Hardware supported virtualization will be a pillar of green IT by enabling extreme server consolidation. Business enterprises have been able to deploy 10:1 or more ratios of decreasing size of their physical servers, which have made significant savings in terms of power consumption, cooling needs, and physical space. This does not only reduce the costs but also significantly reduces the carbon footprint of the technology sector.
Is Hardware-Assisted Virtualization Safe?
This is a vital issue, especially in cloud computing settings where the paradigm of multi-tenancy (i.e., many customers using the same hardware) is the new concept. The brief response is in the affirmative: it is essentially safe.
The model of security is based on the strict isolation of the hardware. The root mode uses the hypervisor as the final gatekeeper. The memory space of one virtual machine cannot be accessed by the memory space of another virtual machine, since the CPU has not been virtualized with the ability to do this. The hardware therefore ensures that the virtual machines are secluded to each other just like they were in different physical networks.
Moreover, cloud providers increase this foundation by adding more security layers which entail virtual firewalls, encryption, and more stringent identity-and-access-management (IAM) policies. These additional security measures are built on a strong and trusted foundation of the hardware when the hardware foundation is in place.
Enumeration of Hardware-Assisted Virtualization
Most probably, your laptop or desktop CPU has these capabilities, but they are probably off by default in the BIOS/UEFI options.
To enable it:
- Reboot your computer and get into the BIOS/ UEFI set up (usually by pressing F2, Delete or Esc during its booting).
- Find the Security settings or the CPU.
- Find a choice called either Intel Virtualization Technology (Intel VT-x), AMD-V, SVM Mode or just Virtualization.
- Enable the selected option.
- Exit the BIOS and save the changes.
Hardware virtualization support is required to effectively use modern hypervisor software like VMware Workstation or Oracle VirtualBox, and a prerequisite to using the Windows Subsystem for Linux (WSL 2) or Android emulators.
Final Thoughts: The Invisible Foundation
One of the major, yet the most elusive, 21st century technologies is hardware-assisted virtualization. This is a critical performance concern that has slowed down the adoption of virtualization.
It is fundamentally based on the fact that it removes the complex operation of managing the virtual machines by transfers of the software operation into the silicon of the CPU, thereby creating a foundation that is highly fast and inherently secure. This platform allows cloud vendors to offer an apparently limitlessly scalable pool of available compute resources that is trustworthy, secluded and economical. Next time you start a cloud server in a couple of seconds, take a moment to remember the minute, hardware-assisted miracles that take place in a CPU a billion times a second to make it possible.
