Re: [PATCH 4/4] vl: Prioritize realizations of devices

[Peter Xu]

On Thu, Aug 26, 2021 at 09:43:59AM -0400, Peter Xu wrote:
> > > A simple state machine can track "has IOMMU" state.  It has three states
> > > "no so far", "yes", and "no", and two events "add IOMMU" and "add device
> > > that needs to know".  State diagram:
> > > 
> > >                           no so far
> > >                    +--- (start state) ---+
> > >                    |                     |
> > >          add IOMMU |                     | add device that
> > >                    |                     |  needs to know
> > >                    v                     v
> > >              +--> yes                    no <--+
> > >              |     |   add device that   |     |
> > >              +-----+    needs to know    +-----+
> > > 
> > > "Add IOMMU" in state "no" is an error.
> > 
> > question is how we distinguish "device that needs to know"
> > from device that doesn't need to know, and then recently
> > added feature 'bypass IOMMU' adds more fun to this.
> 
> Maybe we can start from "no device needs to know"? Then add more into it when
> the list expands.
> 
> vfio-pci should be a natural fit because we're sure it won't break any valid
> old configurations.  However we may need to be careful on adding more devices,
> e.g. when some old configuration might work on old binaries, but I'm not sure.

Btw, I think this state machine is indeed bringing some complexity on even
understanding it - it is definitely working but it's not obvious to anyone at
the first glance, and it's only solving problem for vIOMMU.  E.g., do we need
yet another state machine for some other ordering constraints?

>From that POV, I don't like this solution more than the simple "assign priority
for device realization" idea..

-- 
Peter Xu