KVM: x86/mmu: Use huge pages for DAX-backed files

Walk the host page tables to identify hugepage mappings for ZONE_DEVICE
pfns, i.e. DAX pages.  Explicitly query kvm_is_zone_device_pfn() when
deciding whether or not to bother walking the host page tables, as DAX
pages do not set up the head/tail infrastructure, i.e. will return false
for PageCompound() even when using huge pages.

Zap ZONE_DEVICE sptes when disabling dirty logging, e.g. if live
migration fails, to allow KVM to rebuild large pages for DAX-based
mappings.  Presumably DAX favors large pages, and worst case scenario is
a minor performance hit as KVM will need to re-fault all DAX-based
pages.

Suggested-by: Barret Rhoden <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Jason Zeng <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: Liran Alon <[email protected]>
Cc: linux-nvdimm <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

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