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>

arch/x86/kvm/mmu/mmu.c

@@ -3250,7 +3250,7 @@ static int host_pfn_mapping_level(struct kvm_vcpu *vcpu, gfn_t gfn,
 		     PT_DIRECTORY_LEVEL != (int)PG_LEVEL_2M ||
 		     PT_PDPE_LEVEL != (int)PG_LEVEL_1G);
 
-	if (!PageCompound(pfn_to_page(pfn)))
+	if (!PageCompound(pfn_to_page(pfn)) && !kvm_is_zone_device_pfn(pfn))
 		return PT_PAGE_TABLE_LEVEL;
 
 	/*
@@ -3282,8 +3282,7 @@ static int kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, gfn_t gfn,
 	if (unlikely(max_level == PT_PAGE_TABLE_LEVEL))
 		return PT_PAGE_TABLE_LEVEL;
 
-	if (is_error_noslot_pfn(pfn) || kvm_is_reserved_pfn(pfn) ||
-	    kvm_is_zone_device_pfn(pfn))
+	if (is_error_noslot_pfn(pfn) || kvm_is_reserved_pfn(pfn))
 		return PT_PAGE_TABLE_LEVEL;
 
 	slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn, true);
@@ -5912,8 +5911,8 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
 		 * mapping if the indirect sp has level = 1.
 		 */
 		if (sp->role.direct && !kvm_is_reserved_pfn(pfn) &&
-		    !kvm_is_zone_device_pfn(pfn) &&
-		    PageCompound(pfn_to_page(pfn))) {
+		    (kvm_is_zone_device_pfn(pfn) ||
+		     PageCompound(pfn_to_page(pfn)))) {
 			pte_list_remove(rmap_head, sptep);
 
 			if (kvm_available_flush_tlb_with_range())