KVM: x86/mmu: Consult max mapping level when zapping collapsible SPTEs

When zapping SPTEs in order to rebuild them as huge pages, use the new
helper that computes the max mapping level to detect whether or not a
SPTE should be zapped.  Doing so avoids zapping SPTEs that can't
possibly be rebuilt as huge pages, e.g. due to hardware constraints,
memslot alignment, etc...

This also avoids zapping SPTEs that are still large, e.g. if migration
was canceled before write-protected huge pages were shattered to enable
dirty logging.  Note, such pages are still write-protected at this time,
i.e. a page fault VM-Exit will still occur.  This will hopefully be
addressed in a future patch.

Sadly, TDP MMU loses its const on the memslot, but that's a pervasive
problem that's been around for quite some time.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210213005015.1651772-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -5553,8 +5553,8 @@ 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)))) {
+		    sp->role.level < kvm_mmu_max_mapping_level(kvm, slot, sp->gfn,
+							       pfn, PG_LEVEL_NUM)) {
 			pte_list_remove(rmap_head, sptep);
 
 			if (kvm_available_flush_tlb_with_range())
@@ -5574,12 +5574,13 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
 				   const struct kvm_memory_slot *memslot)
 {
 	/* FIXME: const-ify all uses of struct kvm_memory_slot.  */
+	struct kvm_memory_slot *slot = (struct kvm_memory_slot *)memslot;
+
 	write_lock(&kvm->mmu_lock);
-	slot_handle_leaf(kvm, (struct kvm_memory_slot *)memslot,
-			 kvm_mmu_zap_collapsible_spte, true);
+	slot_handle_leaf(kvm, slot, kvm_mmu_zap_collapsible_spte, true);
 
 	if (is_tdp_mmu_enabled(kvm))
-		kvm_tdp_mmu_zap_collapsible_sptes(kvm, memslot);
+		kvm_tdp_mmu_zap_collapsible_sptes(kvm, slot);
 	write_unlock(&kvm->mmu_lock);
 }
 

arch/x86/kvm/mmu/tdp_mmu.c

@@ -1328,8 +1328,10 @@ bool kvm_tdp_mmu_slot_set_dirty(struct kvm *kvm, struct kvm_memory_slot *slot)
  */
 static void zap_collapsible_spte_range(struct kvm *kvm,
 				       struct kvm_mmu_page *root,
-				       gfn_t start, gfn_t end)
+				       struct kvm_memory_slot *slot)
 {
+	gfn_t start = slot->base_gfn;
+	gfn_t end = start + slot->npages;
 	struct tdp_iter iter;
 	kvm_pfn_t pfn;
 	bool spte_set = false;
@@ -1348,8 +1350,8 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
 
 		pfn = spte_to_pfn(iter.old_spte);
 		if (kvm_is_reserved_pfn(pfn) ||
-		    (!PageCompound(pfn_to_page(pfn)) &&
-		     !kvm_is_zone_device_pfn(pfn)))
+		    iter.level >= kvm_mmu_max_mapping_level(kvm, slot, iter.gfn,
+							    pfn, PG_LEVEL_NUM))
 			continue;
 
 		tdp_mmu_set_spte(kvm, &iter, 0);
@@ -1367,7 +1369,7 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
  * be replaced by large mappings, for GFNs within the slot.
  */
 void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
-				       const struct kvm_memory_slot *slot)
+				       struct kvm_memory_slot *slot)
 {
 	struct kvm_mmu_page *root;
 	int root_as_id;
@@ -1377,8 +1379,7 @@ void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
 		if (root_as_id != slot->as_id)
 			continue;
 
-		zap_collapsible_spte_range(kvm, root, slot->base_gfn,
-					   slot->base_gfn + slot->npages);
+		zap_collapsible_spte_range(kvm, root, slot);
 	}
 }
 

arch/x86/kvm/mmu/tdp_mmu.h

@@ -35,7 +35,7 @@ void kvm_tdp_mmu_clear_dirty_pt_masked(struct kvm *kvm,
 				       bool wrprot);
 bool kvm_tdp_mmu_slot_set_dirty(struct kvm *kvm, struct kvm_memory_slot *slot);
 void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
-				       const struct kvm_memory_slot *slot);
+				       struct kvm_memory_slot *slot);
 
 bool kvm_tdp_mmu_write_protect_gfn(struct kvm *kvm,
 				   struct kvm_memory_slot *slot, gfn_t gfn);