I know Hyper-V is type 1 or native hypervisor, meaning it sits on top of hardware and doesn't require an operating system, i.e. talks to the hardware through the ISA interface).
But I don't understand why does it require hardware assisted virtualization? Does it mean Hyper-V is not full native hypervisor because it requires another part (put in hardware)? Does every native hypervisor require hardware virtualization?
Because without hardware virtualization it would have to run an emulation which comes with a BRUTAL performance implication. There is no way to do proper virtualiaztion without either interpreting a significant number of machine code, or have hardware support for this. EVERY native hypervisor requires hardware virtualization - which is, btw., nothing new... it was in the firstp rocessors mit 60s, iirc (196x). Yes, this is that old. VMS - the Mainframe operating system - is acutally short for... "Virtual Machine System". The processors back then had hardware virtualization.
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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.
I hear a lot about "Hypervisors are not emulators. If you need to emulate another hardware specifications than you computer have, you need to use emulator, not hypervisor".
Well, but yesterday I saw this video on youtube - click here - which shows how to install Win 95 on modern macOS with VMware Fusion.
The strange thing for me is that on 17:39 you could see that Win 95 virtual machine is "Pentium Pro with 64 MB RAM".
Hmm! So, Fusion somehow faked processor and RAM, right? But it is not emulator, right? So, does it mean that any hypervisor can fake processor and RAM?
At the time of its release, Windows 95 only had code to recognize CPUIDs up to Pentium Pro. Any processor not lower than Pentium Pro is "called" Pentium Pro.
The main difference is the Hypervisor cannot emulate CPU code. All code must run on the original processor.
The hypervisor does emulate the BIOS, which in tells the OS the hardware specs available; including RAM, Boot order and peripherals attached.
When you are talking about VMWare Fusion the way this works depends on how virtualization is achieved. According to wikipedia VMWare Fusion utilizes hardware-assisted virtualization, dynamic binary translation, and para-virtualization.
In the hardware-assited virtualization case, #Strom is correct and guest instructions can be executed directly on the host CPU. Besides #Strom answer, you can fake the CPU type by trapping and emulating the cpuid instruction.
In the para-virtualization case you replace critical instructions by calls to the hypervisor which emulates the instruction on behalf of the guest. So again you emulate the cpuid instruction to "fake" the CPU type. Keep in mind that this requires a modified, hence para-virtualized, guest operating system.
Finally, dynamic binary translation scans the guest code for critical instructions during runtime and either replaces them by traps into the hypervisor achieving some kind of "live para-virtualization" or translating blocks of guest code into equivalent blocks of host code that modifies the VM state according to the original guest code (this is e.g. how the QEMU full system emulator works). As a result, again you are able to "fake" the CPU type by emulating the cpuid instruction. Notice that guest and host can be the same architecture in this case, but there is no need for this.
Of course a combination of above techniques is also feasible.
As for virtualization of main memory, the hypervisor is in full control of the hardware so you can simply configure a VM with just 64MB of main memory. The VM is not able to "see" more than this due to the techniques shortly discussed above.
Please keep in mind that this just gives a very short overview of virtualization and I tried to keep it short and informative, so I know my explanations are partially not very accurate. If you are really interested in virtualization I recommend reading "Virtual Machines: Versatile Platforms for Systems and Processes" or the papers on the topic by Popek & Goldberg and "Xen and the Art of Virtualization"
What actually happens when hardware virtualization is enabled ?
if not, a hypervisor uses binary translation. but, when hardware virtualization is enabled, i have read that it uses trap and emulate.
so the guest code executes directly on the host cpu, if its a privileged instruction the cpu hands over the control to the hypervisor, the hypervisor emulates that instruciton and then executes it.
so, what does the emulation means here ? is the same binary translation carried out when hardware virtualization is enable ?
Enabling HW virtualization sets the vmx flag in Intel and svm flag in AMD.
In Intel architecture, this allows the user-space calls to run as-is on the lower protection ring as they cannot potentially interfear with the host OS. On the other hand the kernel-space calls of the virualized OS are trapped and binary-translated by the hypervisor.
This is done so as to partly take away the CPU intensive translation for trivial calls. How-much of this happens depends on the virtualization type- full, partial or paravirtual.
Binary-translation is a subset of the more elaborate process of emulation. It alligns the guest code to be able to run on the host-architecture.
If hardware support is a must for virtualization, how can Java Virtual Machines run on machines without support for virtualization ? Or is JVM not a virtual machine ?
A JVM is not virtual in the same sense as a VirtualBox or VMWare virtual machine. It is a 'machine' that implements the Java bytecode, not a virtualized version of actual hardware.
The term-of-art 'virtual machine' was coined a very long time ago for the following scenario:
make up a computer, like Knuth's MIX.
write a computer program that implements the made-up computer.
run programs
When this virtual machine runs, it's a completely ordinary program, running completely in user mode. It needs no special help from the hardware or operating system to work reasonably well. This is especially true of the JVM, since the Java byte code does not deal with low-level hardware I/O or other things which are hard to simulate.
Later, historically, (to pick a particular instance), IBM invented VM/370. VM/370 uses the other sense of the term 'virtual machine'. In this later sense, the hardware and operating system cooperate to allow a single physical machine to host multiple virtual instances of (more or less) the same architecture, in which multiple copies of the whole operating system are written as if they are running on more or less bare hardware. Later, the X86 was designed with features to facilitate this.
So, yes, any virtual machine is making use of some physical hardware, unless you implement it with pieces of paper passed around a table (pace John Searle). But when the virtual machine bears no resemblance to the machine it is running on, then there's no need for special help from the operating system and hardware, and no need for anything as complex as VM/370, or VMware.
If hardware support is a must for virtualization, ...
Let me stop you right there :-)
There is a difference in concept between the JVM (software virtualization) and (for example) a VMWare VM (hardware-assisted virtualization).
The JVM (and other software-based VMMs such as the ones that allow to to emulate x86 on Solaris hardware - I think Bochs and possibly DosBox fall into this category) runs like any other application, using the operating system to gain access to the hardware, or emulating its own hardware purely in software.
VMWare, and the other VMMs optimised for speed, rely on hardware support. In other words, they run on the hardware as if they have full access to the hardware and, only when they try to do something they're not supposed to does the OS captures that attempt and fake it.
That's why VMWare runs so much faster than the software-only emulators. It's because, for the vast majority of the time, it's actually running on the real hardware.
The JVM is a virtual machine, but it doesn't require any additional support from the Operating System. Instead of virtualising instructions for a particular CPU it executes java bytecode.
The JVM is a virtual machine for running Java, in other words it emulates a machine which would be capable of running java. It is a confusing choice of names, but it comes from the general meaning of "machine" not from the more common Virtual Machine meaning.
The JVM, like a regular VM emulates the execution of instructions, but in the case of the JVM the instructions being emulated are Java Instructions, and in the case of a VM they are Hardware Instructions as would be executed by an OS running on the same hardware.
Yes the JVM does access hardware, however this is why you install a MAC or WINDOWS JVM since the instructions are translated by the JVM and acted upon depending on the installation of the JVM, for example, open file dialog on mac opens the mac dialog and windows JVM opens the windows dialog.
So its not being virtualized by the system, but the bytecode is being virtualized by the JVM you installed. It's basically like an application that reads something(bytecode) and does something(access hardware, or other stuff).
It should be noted that nothing stipulates that a JVM does not (have to) have HW virtualization access. There are notable exceptions, but to which the answered poster alluded, few CPs exist that run Java bytecode natively. Maybe someday a Java bytecode HAL or TIMI will be commonplace to put the JVM into the same class as the formalized HW virtualization?
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.