Ok, here's a "final" patch set to enable RC6 where possible on SNB and IVB machines.
The first patch creates a new variable, intel_iommu_enabled, that is exported by the intel iommu code and set when that code has successfully initialized itself. The old plan of using no_iommu || dmar_disabled would work -- those variables are set only by kernel parameters and don't reflect what the system is actually doing about virtualization.
The second patch uses that value on SNB to tell whether RC6 can be enabled by default. On IVB, RC6 is always enabled.
Of course, in all cases, you can override the RC6 setting with the i915 module parameter.
For those of you who have experienced the delights of RC6 crashing your machines, please test as this will be heading to 3.2 unless you find something wrong with it.
-keith