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

[Arne Vajhøj]

On 5/15/2023 8:53 AM, Johnny Billquist wrote:
> On 2023-05-15 14:36, Arne Vajhøj wrote:
>> On 5/15/2023 8:20 AM, Simon Clubley wrote:
>>> On 2023-05-12, Arne Vajhøj <arne at vajhoej.dk> wrote:
>>>> On 5/12/2023 1:30 PM, Simon Clubley wrote:
>>>>> On 2023-05-12, Dave Froble <davef at tsoft-inc.com> wrote:
>>>>>> On 5/12/2023 8:14 AM, Simon Clubley wrote:
>>>>>>> That's going to make for some "interesting" real-time program 
>>>>>>> behaviour... :-)
>>>>>>
>>>>>> Do you think any serious real time programmer will run a real time 
>>>>>> task inside a
>>>>>> VM?  I'm not a real time programmer, and I'd still not do that.
>>>>>
>>>>> As well as traditional real-time stuff (which I agree with you 
>>>>> about BTW),
>>>>
>>>> I would not want to do it on a type 2 hypervisor - there must be
>>>> cases where what is happening on the host OS impact the performance
>>>> of the guest OS.
>>>>
>>>> But with a type 1 hypervisor and no over allocation of resources -
>>>> would it be worse than running on bare metal?
>>>
>>> A type 1 hypervisor is still a software layer between the hardware and
>>> the RTOS. It would have to _guarantee_ that it would _never_ get in the
>>> way of the timing guarantees that a program running under a RTOS needs.
>>
>> It is software that is active between the VMS and the HW.
>>
>> But it is not obvious to me where any delay would come from.
>>
>> If we talk type 2 then it is easy to understand. You have a host
>> OS running 200 processes - 1 VM and 199 other stuff, and 4 CPU
>> so those 200 processes get scheduled on those 4 CPU's, and certain
>> interrupts may happen in the host OS. That VM will have
>> a problem providing real time capabilities.
>>
>> But in the type 1 with no over allocation of resources then
>> what will cause any delays? The VM got its own CPU or CPU's
>> that no other VM want, so I would expect the hypervisor to
>> keep them permanently allocated to the VM. And I don't
>> see the need for any interrupts in the hypervisor either.
> 
> Can you guarantee that no interrupts happens on the CPUs the hypervisor 
> is using? I would suspect "no", which means you do not have full control.

What would those interrupts do?

A type 1 hypervisor does not do that much.

It has not very much in common with a type 2 hypervisor. I see it
as more of a software equivalent of system partitioning (DEC Galaxy,
IBM LPAR).

Arne