iommu hypervisor hypothetical

[John Rose]

Suppose the hypervisor added new failure codes for the hcall that does
tce_build().  Suppose that the device driver needed to expect such
failures when alloc_[coherent,consistent] is not successful, and handle
that case accordingly.

Is this something we can accommodate without rewriting the iommu stuff
for various platforms?  All these functions in machdep.h are defined to
return NULL.  Is this because we haven't had a reasonable expectation of
failure, or because device drivers _have_ to assume success for such
operations?

Thanks-
John