KVM: Write protect the updated slot only when dirty logging is enabled

Calling kvm_mmu_slot_remove_write_access() for a deleted slot does
nothing but search for non-existent mmu pages which have mappings to
that deleted memory; this is safe but a waste of time.

Since we want to make the function rmap based in a later patch, in a
manner which makes it unsafe to be called for a deleted slot, we makes
the caller see if the slot is non-zero and being dirty logged.

Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

arch/x86/kvm/x86.c

@@ -6897,7 +6897,13 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 	spin_lock(&kvm->mmu_lock);
 	if (nr_mmu_pages)
 		kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
-	kvm_mmu_slot_remove_write_access(kvm, mem->slot);
+	/*
+	 * Write protect all pages for dirty logging.
+	 * Existing largepage mappings are destroyed here and new ones will
+	 * not be created until the end of the logging.
+	 */
+	if (npages && (mem->flags & KVM_MEM_LOG_DIRTY_PAGES))
+		kvm_mmu_slot_remove_write_access(kvm, mem->slot);
 	spin_unlock(&kvm->mmu_lock);
 	/*
 	 * If memory slot is created, or moved, we need to clear all

virt/kvm/kvm_main.c

@@ -817,7 +817,6 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	if ((new.flags & KVM_MEM_LOG_DIRTY_PAGES) && !new.dirty_bitmap) {
 		if (kvm_create_dirty_bitmap(&new) < 0)
 			goto out_free;
-		/* destroy any largepage mappings for dirty tracking */
 	}
 
 	if (!npages || base_gfn != old.base_gfn) {