KVM: introduce id_to_memslot function

Introduce id_to_memslot to get memslot by slot id

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 917a287..b6776c6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3520,7 +3520,7 @@
 	if (log->slot >= KVM_MEMORY_SLOTS)
 		goto out;
 
-	memslot = &kvm->memslots->memslots[log->slot];
+	memslot = id_to_memslot(kvm->memslots, log->slot);
 	r = -ENOENT;
 	if (!memslot->dirty_bitmap)
 		goto out;
@@ -3531,27 +3531,27 @@
 	/* If nothing is dirty, don't bother messing with page tables. */
 	if (nr_dirty_pages) {
 		struct kvm_memslots *slots, *old_slots;
-		unsigned long *dirty_bitmap;
+		unsigned long *dirty_bitmap, *dirty_bitmap_head;
 
-		dirty_bitmap = memslot->dirty_bitmap_head;
-		if (memslot->dirty_bitmap == dirty_bitmap)
-			dirty_bitmap += n / sizeof(long);
-		memset(dirty_bitmap, 0, n);
+		dirty_bitmap = memslot->dirty_bitmap;
+		dirty_bitmap_head = memslot->dirty_bitmap_head;
+		if (dirty_bitmap == dirty_bitmap_head)
+			dirty_bitmap_head += n / sizeof(long);
+		memset(dirty_bitmap_head, 0, n);
 
 		r = -ENOMEM;
 		slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
 		if (!slots)
 			goto out;
 		memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
-		memslot = &slots->memslots[log->slot];
-		memslot->dirty_bitmap = dirty_bitmap;
+		memslot = id_to_memslot(slots, log->slot);
 		memslot->nr_dirty_pages = 0;
+		memslot->dirty_bitmap = dirty_bitmap_head;
 		update_memslots(slots, NULL);
 
 		old_slots = kvm->memslots;
 		rcu_assign_pointer(kvm->memslots, slots);
 		synchronize_srcu_expedited(&kvm->srcu);
-		dirty_bitmap = old_slots->memslots[log->slot].dirty_bitmap;
 		kfree(old_slots);
 
 		write_protect_slot(kvm, memslot, dirty_bitmap, nr_dirty_pages);