(In reply to Christian König from comment #23)
> > How would it break backwards compatibility?
> 
> You would need to allow multiple mappings into the same address space per BO.
> 
> Which is exactly what I've did for amdgpu, but IIRC that would break the
> userspace interface because you won't return the mapped address any more
> when you try to map it multiple times....

What would that break? It could result in the same BO having several
representations in userspace, but (why) is that a problem?

> > I'm not sure how not tracking the VA ranges per GEM handle could ever work
> > as expected with several GEM handles referencing the same BO.
> 
> Actually it can indeed never work correctly. What we just do all the time is
> trying to avoid the case that several GEM handles reference the same BO very
> hard.

I'm afraid we can't always avoid that though, e.g. when sharing BOs between
glamor and the Xorg driver.