Lots of use cases

Posted Sep 21, 2025 4:37 UTC (Sun) by kazer (subscriber, #134462)
In reply to: Lots of use cases by geofft
Parent article: Multiple kernels on a single system

> two different kernels

That second "foreign" kernel would need to understand the "partition" it is allowed to use so it won't try to take over rest of the machine where another kernel may be running. Unless there is a way to make hardware understand where that another one is allowed to run (basically selective removing of supervisor-rights from the foreign kernel).

So I can only see that happening if the second kernel understands multikernel situations correctly as well. Otherwise it is back to hypervisor virtualization.

> old kernel

Sorry, but for the reasons mentioned above (supervisor access to hardware) that old kernel would need to be multikernel compliant as well. Otherwise you need a plain old hypervisor for virtualization.

Lots of use cases

Posted Sep 21, 2025 12:16 UTC (Sun) by kleptog (subscriber, #1183) [Link] (3 responses)

> That second "foreign" kernel would need to understand the "partition" it is allowed to use so it won't try to take over rest of the machine where another kernel may be running

It is already the case that a booting kernel asks the underlying system which part of physical memory it is allowed to use. It can then prepare the kernel mapping so it can only access the parts it is allowed to. It can't assume anything about all the other parts.

Now, this only prevents accidental interference. There's nothing that prevents the kernel from modifying its mapping (dynamically adding RAM/devices is a thing) but it would give a very high degree of isolation. Not as good as a hypervisor, but pretty good.

Lots of use cases

Posted Sep 21, 2025 17:46 UTC (Sun) by glettieri (subscriber, #15705) [Link] (2 responses)

> It is already the case that a booting kernel asks the underlying system which part of physical memory it is allowed to use

However, in this case the underlying system is the hardware, that doesn't know anything about these partitions. A non-multikernel-aware kernel would discover all the memory and all the devices, and think that it owns everything.

Lots of use cases

Posted Sep 22, 2025 4:50 UTC (Mon) by skissane (subscriber, #38675) [Link] (1 responses)

> However, in this case the underlying system is the hardware, that doesn't know anything about these partitions. A non-multikernel-aware kernel would discover all the memory and all the devices, and think that it owns everything.

Maybe someone just needs to add a “telling lies facility” to the hardware/firmware which the multikernel could use to get the hardware/firmware to lie to the non-multikernel-aware kernel? This could be much more lightweight than standard virtualisation since it wouldn’t be involved at runtime only in config discovery

Lots of use cases

Posted Sep 22, 2025 22:13 UTC (Mon) by Wol (subscriber, #4433) [Link]

And then the non-multi-kernel-aware kernel trips over a bug, tries to do something which would normally crash, and there just happens to be something real there that it accidentally trashes ...

Cheers,
Wol