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

[Arne Vajhøj]

On 5/15/2023 4:47 PM, Dan Cross wrote:
> In article <u3tv4e$34ma3$1 at dont-email.me>,
> Arne Vajhøj  <arne at vajhoej.dk> wrote:
>> On 5/15/2023 1:47 PM, Johnny Billquist wrote:
>>> On 2023-05-15 15:35, Arne Vajhøj wrote:
>>>> On 5/15/2023 8:51 AM, Johnny Billquist wrote:
>>>>> Um? What is the hypervisor then?
>>>>
>>>> The hypervisor software like ESXi.
>>>>
>>>> It is not running on top of a host OS like a type 2 (VirtualBox etc.)
>>>> does.
>>>
>>> I never assumed you had a full OS. However, they Hypervisor is
>>> intercepting access to hardware, and is responsible for the actual
>>> interaction with the hardware, adding a layer between the guest OS and
>>> the hardware.
>>
>> The hypervisor is there.
>>
>> But no host OS.
> 
> 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.

Arne