[Info-vax] clock problems with OpenVMS x86 on VirtualBox

[Arne Vajhøj]

On 5/17/2023 8:25 PM, Arne Vajhøj wrote:
> On 5/15/2023 4:47 PM, Dan Cross wrote:
>> For all intents and purposes, the hypervisor _is_ the host OS.
>> Some software component must drive the hardware, run VCPUs, etc:
>> that's the hypervisor.
> 
> A hypervisor is not a host OS.
> 
> Not in terminology and not in functionality.
> 
> The stacks are:
> 
> guest OS
> type 1 hypervisor
> hardware
> 
> and:
> 
> guest OS
> type 2 hypervisor
> host OS
> hardware
> 
> Host OS and hypervisors are terms used for different things.
> 
> And used for things that provide different functionality:
> 
> a host OS is a normal OS like Linux or Windows able to run
> applications in general including but certainly not limited
> to type 2 hypervisors
> 
> a type 1 hypervisor like VMWare ESXi is a special piece of
> software able to run VM's and partition the hardware resources
> between the VM's - usually they are extremely thin and with only
> this very specialized functionality - any heavy stuff
> are usually put elsewhere in a VM (often known as a Dom0
> VM) or a remote management application
> 
> a type 2 hypervisor like VMWare Player is an application running
> on a host OS that runs a VM
> 
>> The arbitrary distinction between "Type-1" and "Type-2" style
>> hypervisors is also a bit illusory among those who work on them;
>> like all approximations, it is just that.
> 
> Anyone that works with them will know the difference between type
> 1 and type 2 (and the difference between a hypervisor and a host OS).
> 
> It is not an approximation. It is distinct as the difference
> between 3 and 4.

There is one hypervisor that tend to muddy discussions: KVM.

KVM comes with Linux but is a type 1 hypervisor.

If people think of it as:

guest OS
KVM
Linux
hardware

then KVM looks like a type 2 hypervisor.

But KVM is not a Linux application - KVM is part of the Linux kernel.

So the correct picture is:

guest OS
Linux with KVM
hardware

so it is a type 1 hypervisor combined with an OS.

But it is not a thin hypervisor not doing anything else than
running VM's and managing hardware.

This sub-thread started about running real time stuff in a VM. And
I would not put KVM on the top of my hypervisor list for real time
VM's, because of the fact that it comes with an entire Linux
as part of the hypervisor. But the KVM people consider it doable.
They have guidelines on how to setup KVM for VM's running real time
Linux with real time applications.

Arne