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

[Igor Mammedov]

On Thu, 26 Aug 2021 10:01:01 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> Peter Xu <peterx@redhat.com> writes:
> 
> > On Wed, Aug 25, 2021 at 05:50:23PM -0400, Peter Xu wrote:  
> >> On Wed, Aug 25, 2021 at 02:28:55PM +0200, Markus Armbruster wrote:  
> >> > Having thought about this a bit more...
...
> > Any further thoughts will be greatly welcomed.  
> 
> I'd like to propose a more modest solution: detect the problem and fail.
That's or proper dependency tracking (which stands chance to work with QMP,
but like it was said it's complex)

> 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.
 
> "Add IOMMU" in state "yes" stays in state "yes" if multiple IOMMU make
> sense.  Else it's an error.
> 
> The state machine could be owned by the machine type.
> 
>