KVM: x86/mmu: Shove refcounted page dependency into host_pfn_mapping_level()

Move the check that restricts mapping huge pages into the guest to pfns
that are backed by refcounted 'struct page' memory into the helper that
actually "requires" a 'struct page', host_pfn_mapping_level().  In
addition to deduplicating code, moving the check to the helper eliminates
the subtle requirement that the caller check that the incoming pfn is
backed by a refcounted struct page, and as an added bonus avoids an extra
pfn_to_page() lookup.

Note, the is_error_noslot_pfn() check in kvm_mmu_hugepage_adjust() needs
to stay where it is, as it guards against dereferencing a NULL memslot in
the kvm_slot_dirty_track_enabled() that follows.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220429010416.2788472-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -2791,8 +2791,8 @@ static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep)
 static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 				  const struct kvm_memory_slot *slot)
 {
-	struct page *page = pfn_to_page(pfn);
 	int level = PG_LEVEL_4K;
+	struct page *page;
 	unsigned long hva;
 	unsigned long flags;
 	pgd_t pgd;
@@ -2800,6 +2800,14 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 	pud_t pud;
 	pmd_t pmd;
 
+	/*
+	 * Note, @slot must be non-NULL, i.e. the caller is responsible for
+	 * ensuring @pfn isn't garbage and is backed by a memslot.
+	 */
+	page = kvm_pfn_to_refcounted_page(pfn);
+	if (!page)
+		return PG_LEVEL_4K;
+
 	if (!PageCompound(page) && !kvm_is_zone_device_page(page))
 		return PG_LEVEL_4K;
 
@@ -2884,7 +2892,7 @@ void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 	if (unlikely(fault->max_level == PG_LEVEL_4K))
 		return;
 
-	if (is_error_noslot_pfn(fault->pfn) || !kvm_pfn_to_refcounted_page(fault->pfn))
+	if (is_error_noslot_pfn(fault->pfn))
 		return;
 
 	if (kvm_slot_dirty_track_enabled(slot))
@@ -5996,7 +6004,7 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
 		 * the guest, and the guest page table is using 4K page size
 		 * mapping if the indirect sp has level = 1.
 		 */
-		if (sp->role.direct && kvm_pfn_to_refcounted_page(pfn) &&
+		if (sp->role.direct &&
 		    sp->role.level < kvm_mmu_max_mapping_level(kvm, slot, sp->gfn,
 							       pfn, PG_LEVEL_NUM)) {
 			pte_list_remove(kvm, rmap_head, sptep);

arch/x86/kvm/mmu/tdp_mmu.c

@@ -1750,10 +1750,6 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
 		 * be mapped at a higher level.
 		 */
 		pfn = spte_to_pfn(iter.old_spte);
-
-		if (!kvm_pfn_to_refcounted_page(pfn))
-			continue;
-
 		max_mapping_level = kvm_mmu_max_mapping_level(kvm, slot,
 				iter.gfn, pfn, PG_LEVEL_NUM);