Re: [RFC] proposal: KVM: Orphaned VMs: The Caretaker approach for Live Update

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On 04-29 09:40, David Woodhouse wrote:
> On Wed, 2026-04-29 at 10:13 +0200, Alexander Graf wrote:
> > I would prefer we only attach the whole caretaker and all of its 
> > specialties right around the point when live update happens. Why keep it 
> > dangling and active forever? That way you can also late load the kernel 
> > module that contains it, so you can be sure it's an up to date version.
> 
> "Why keep it dangling and active forever?"
> 
> I've always wanted to tie this to address space isolation.
> 
> The only way to truly stay in front of the constant stream of new
> speculation vulnerabilities has been to just make sure there's nothing
> sensitive accessible in the address space at all. Hence all the work on
> secret hiding, XPFO, proclocal, etc. — and hence the occasional
> researcher finding their shiny new (5-year-old) vulnerability and being
> confused when it doesn't leak anything *interesting* in certain
> environments.
> 
> I'd like to see the inner KVM_RUN loop switch to a completely separate
> address space, in which there's a kind of caretaker which can handle
> the bare minimum of interrupts and timers and the most common exits,
> and which *relatively* rarely has to come back into the real Linux
> address space.
> 
> And once you have that caretaker running in its own address space...
> why not just let it keep going while Linux does its kexec?

Yep, this captures one of the benefits of having a permanently attached
Caretaker.

By establishing that isolated execution environment for the inner
KVM_RUN loop to mitigate speculation vulnerabilities, we naturally get
the hardware-enforced boundary required to survive the kexec gap. The
Live Update capability is effectively a byproduct of achieving true
Address Space Isolation.

+CC KP and Brendan