KVM: Allow browsing memslots with mmu_lock

This allows reading memslots with only the mmu_lock hold for mmu
notifiers that runs in atomic context and with mmu_lock held.
Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/x86.c

@@ -3974,16 +3974,23 @@ int kvm_arch_set_memory_region(struct kvm *kvm,
 	 */
 	if (!user_alloc) {
 		if (npages && !old.rmap) {
+			unsigned long userspace_addr;
+
 			down_write(&current->mm->mmap_sem);
-			memslot->userspace_addr = do_mmap(NULL, 0,
+			userspace_addr = do_mmap(NULL, 0,
 						 npages * PAGE_SIZE,
 						 PROT_READ | PROT_WRITE,
 						 MAP_SHARED | MAP_ANONYMOUS,
 						 0);
 			up_write(&current->mm->mmap_sem);
 
-			if (IS_ERR((void *)memslot->userspace_addr))
-				return PTR_ERR((void *)memslot->userspace_addr);
+			if (IS_ERR((void *)userspace_addr))
+				return PTR_ERR((void *)userspace_addr);
+
+			/* set userspace_addr atomically for kvm_hva_to_rmapp */
+			spin_lock(&kvm->mmu_lock);
+			memslot->userspace_addr = userspace_addr;
+			spin_unlock(&kvm->mmu_lock);
 		} else {
 			if (!old.user_alloc && old.rmap) {
 				int ret;

virt/kvm/kvm_main.c

@@ -375,7 +375,15 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		memset(new.rmap, 0, npages * sizeof(*new.rmap));
 
 		new.user_alloc = user_alloc;
+		/*
+		 * hva_to_rmmap() serialzies with the mmu_lock and to be
+		 * safe it has to ignore memslots with !user_alloc &&
+		 * !userspace_addr.
+		 */
+		if (user_alloc)
 			new.userspace_addr = mem->userspace_addr;
+		else
+			new.userspace_addr = 0;
 	}
 	if (npages && !new.lpage_info) {
 		int largepages = npages / KVM_PAGES_PER_HPAGE;
@@ -408,17 +416,21 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	}
 #endif /* not defined CONFIG_S390 */
 
-	if (mem->slot >= kvm->nmemslots)
-		kvm->nmemslots = mem->slot + 1;
-
 	if (!npages)
 		kvm_arch_flush_shadow(kvm);
 
+	spin_lock(&kvm->mmu_lock);
+	if (mem->slot >= kvm->nmemslots)
+		kvm->nmemslots = mem->slot + 1;
+
 	*memslot = new;
+	spin_unlock(&kvm->mmu_lock);
 
 	r = kvm_arch_set_memory_region(kvm, mem, old, user_alloc);
 	if (r) {
+		spin_lock(&kvm->mmu_lock);
 		*memslot = old;
+		spin_unlock(&kvm->mmu_lock);
 		goto out_free;
 	}