I always wondered about virtualization environments which have pageable
guest memory - how does the guest kernel handle situations where it
really needs non-pageable memory? Does is simply "not care" because for
it the memory access looks just like it isn't paged but is simply very,
very slow?

Yes - from the guest's point of view, what it thinks is physical memory appears wired.

What about time-sensitive situations (like the originally
mentioned PCI-passthrough)?

PCI passthru is a special case since the h/w requires that the target of a DMA transfer is present - there is currently no way for the IOMMU to generate the equivalent of page faults. So, for PCI passthru, all of guest memory has to be wired host memory.

For time-sensitive situations, there's not a lot of options other than forcing guest memory to be wired, since there isn't visibility into the host without having o/s-specific "tools" that could communicate this information to the hypervisor.



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