How does a Hypervisor distinguish multiple VMs running on it and isolate them from the underlying h/w?
e.g. if there is a system call from with in a guest OS, how does the HV know it belongs to a specific guest OS?
Not much details about lower details of HV operation.
A normal system call in a guest is processed by the guest OS without intervention of the hypervisor.
However, when the guest does cause a trap to the hypervisor (not a system call, but some other operation that requires hypervisor service), the hypervisor knows which guest it is because it knows which guest it scheduled on that CPU.
Related
When we work with type 2 hypervisors it is very easy to say which OS is the main one. For example, if you install some type 2 hypervisor on Win 7, and launch Win 95 inside this hypervisor, the main OS will be Win 7. The conception is obvious.
However, it's not so obvious with type 1 hypervisors. I never worked with them before.
You have few operating systems on top the hypervisor. So... Which one of these OSs will be the main one? How this question is resolved? And probably (just a guess) there is no such thing as "main OS" in this case?
I don't think that "main" operating system is a defined term.
A type 2 hypervisor is an extension to an operating system, which is known as the host operating system when guest operating systems are running on top of it. A host operating system runs directly on the hardware and needs to have specific code to interact with the hardware (e.g. the NIC, the disk, etc.) and provide abstractions to user-level programs. The hypervisor simply extends the functionality of the host operating system to allow guest operating systems to run on top (e.g. when the guest operating system wants to write to the hard drive, the hypervisor translates this request to a form that the host OS can understand so that the host OS can make the disk access).
A type 1 hypervisor runs directly on the hardware without an operating system. A type 1 hypervisor is basically just a stripped down operating system with the functionality necessary to allow guest operating systems to run on top. When the guest needs to write to disk or do some other privileged operation, the type 1 hypervisor receives the request and acts on it. Perhaps the type 1 hypervisor is what you would consider the "main" OS? Regardless, I would avoid using that term.
I would argue that the "main" OS would be the Hypervisor software itself, as it runs directly on the hardware and supports the virtual operating systems, as well as boots on system startup.
How can an application detect when the hypervisor has resumed its guest OS after suspending it due to, for example, migration? Is there an API for it, depending on the specific hypervisor, or is there something in the environment (Linux or Windows) that unambiguously indicates resumption?
There's not really anything that I can think of off the top of my head. The guest OS, for the most part, doesn't really know it's virtualized so it doesn't have a concept of what host it's on or whether or not it's being migrated and/or suspended at the hypervisor level.
Is there a particular use case you can share where this would be helpful?
Suppose in my pc I have Ubuntu as Host OS. Now I installed a Virtual Machine say VirtualBox (hypervisor) and then deployed a centos and a redhat os inside that as guest OS.
Suppose CentOS and redhat has 2 processes running and Ubuntu is running 3 processes. So following are my questions:
There are how many processes that Ubuntu is having?
Is there any difference between GuestOS and HostOS processes?
If all guestos runs as a process then they will get less time as compared to other process running on host os.
Please clear my doubts here.
Thank you.
Well let me clear your doubts,
First of all there aren't any specific number of process for an OS, its called as cores or threads, technically you can define how many cores or threads you want to use on your virtual machine and it depends on the system configuration you use.
Secondly Guest OS is what you have created in the virtual machine and host is what your laptop or pc actually run. Host OS uses the actual hardware for the working whereas the Guest OS uses the virtual hardware like number of cores and type and size of hard drive defined by the user while adding a virtual machine.
Third, as I mentioned earlier Guest and Host OS works on the configurations used by you, if you user higher amount of cores/ threads in setting your virtual machine the Guest OS will get higher speed.
Ideally the virtual machines are used to test and create some functionality of the Operating Systems without affecting the internal OS, so you can think of it as a your parents house where you can live and grow but at the end you cannot go away from the fact that their contribution is more and so you cannot go beyond their features without leaving it and making your own home.
Linux operating systems are multi-threaded operating system. The host OS would consider virtual box as a thread. You can define number of cores and virtual hard disk size for guest OS by using virtual box.
Since virtual box runs in separate thread and other operations of host OS runs in separate threads, there would be less effect on speed of processing. But I've observed big variances in processing speed in systems which have low memory. Each and every thread needs specific allocation of memory for its smooth operation. So systems having more than 2 GB RAM managed virtual box very well.
What's the difference between process virtual machine with system virtual machine?
My guess is that process VM is not providing a kind of an operating system for the whole application for that OS, rather providing an environment for some specific application.
And system VM is providing an environment for an OS to be installed just like VirtualBox.
Am I getting it correct?
Another question is the difference between the two different implementation of system VM: hosted vs. stand-alone.
I'm a beginner studying OS, so easy and understandable answer would be greatly appreciated :)
A Process virtual machine, sometimes called an application virtual machine, runs as a normal application inside a host OS and supports a single process. It is created when that process is started and destroyed when it exits. Its purpose is to provide a platform-independent programming environment that abstracts away details of the underlying hardware or operating system, and allows a program to execute in the same way on any platform.
A System virtual machine provides a complete system platform which supports the execution of a complete operating system (OS),Just like you said VirtualBox is one example.
A Host virtual machine is the server component of a virtual machine , which provides computing resources in the underlying hardware to support guest virtual machine (guest VM).
The following is from http://airccse.org/journal/jcsit/5113ijcsit11.pdf :
System Virtual Machines
A System Virtual Machine gives a complete virtual hardware platform with support for execution
of a complete operating system (OS).
The advantage of using System VM are:
Multiple Operating System environments can run in parallel on the same piece of
hardware in strong isolation from each other.
The VM can provide an instruction set architecture (ISA) that is slightly different from
that of the real machine
The main draw backs are:
Since the VM indirectly accesses the same hardware the efficiency is compromised.
Multiply VMs running in parallel on the same physical machine may result in varied
performance depending on the workload imposed on the system. Implementing proper
isolation techniques may address this drawback.
I'm looking into using virtual machines to host multiple OSes and I'm looking at the free solutions which there are a lot of them. I'm confused by what a hypervisor is and why are they different or better than a "standard" virtual machine. When I mean standard I going to use the benchmark virtual machine VMWare Server 2.0.
For a dual core system with 4 GB of ram that would be capable of running a max of 3 VMs. Which is the best choice? Hypervisor or non-hypervisor and why? I've already read the Wikipedia article but the technical details are over my head. I need a basic answer of what can these different VM flavors do for me.
My main question relates to how I would do testing on multiple environments. I am concerned about the isolation of OSes so I can test applications on multiple OSes at the same time. Also which flavor gives a closer experience of how a real machine operates?
I'm considering the following:
(hypervisor)
Xen
Hyper-V
(non-hypervisor)
VirtualBox
VMWare Server 2.0
Virtual PC 2007
*The classifications of the VMs I've listed may be incorrect.
The main difference is that Hyper-V doesn't run on top of the OS but instead along with the system it runs on top of a thin layer called hypervisor. Hypervisor is a computer hardware platform virtualization software that allows multiple operating systems to run on a host computer concurrently.
Many other virtualization solution uses other techniques like emulation. For more details see Wikipedia.
Disclaimer, everything below is (broadly) my opinion.
Its helpful to consider a virtual machine monitor (a hypervisor) as a very small microkernel. It has very few jobs beyond accessing the underlying hardware, such as monitoring of event channels and granting guest domains access to specific resources .. while enforcing some kind of scheduler.
All guest machines are completely oblivious of the others, the isolation is true. Guests do not share memory with the privileged guest (or each other). So, in this instance, you could (roughly) think of each guest (even the privileged one) as a process, as far as the VMM is concerned. Typically, the first guest gets extra privileges so that it can manage the rest. This is the ideal technology to use when virtual machines are put into production and exposed to the world.
Additionally, some guests can be patched to become aware of the hypervisor, significantly increasing their performance.
On the other hand we have things like VMWare and QEMU, which rely on the host kernel to give it access to bare metal and enough memory to exist. They assume that all guests need to be presented with a complete machine, the limits put on the process presenting these (more or less) become the limits of the virtual machine. I say more or less because device mapper QoS is not commonly implemented. This is the ideal solution for trying code in some other OS, or some other architecture. A lot of people will call QEMU, Simics or even sometimes VMWare (depending on the product) a 'simulator'.
For production roll outs I use Xen, for testing something I just cross compiled I use QEMU, Simics or VirtualBox.
If you are just testing / rolling new code on various operating systems and architectures, I highly recommend #2. If your need is introspection (i.e. watching guest memory change as bad programs run in a guest) ... I'd need more explanation before answering.
Benefits of Hypervisor:
Hypervisor separates virtual machines logically, assigning each its own slice of underlying computing power, memory, and storage, thus preventing the virtual machines from interfering with each other.