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

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 <brho@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jason Zeng <jason.zeng@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Liran Alon <liran.alon@oracle.com>
Cc: linux-nvdimm <linux-nvdimm@lists.01.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>