KVM: arm64: Remove list_head from hyp_page

The list_head member of struct hyp_page is only needed when the page is
attached to a free-list, which by definition implies the page is free.
As such, nothing prevents us from using the page itself to store the
list_head, hence reducing the size of the vmemmap.
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210608114518.748712-4-qperret@google.com

arch/arm64/kvm/hyp/include/nvhe/memory.h

@@ -12,7 +12,6 @@ struct hyp_page {
 	unsigned int refcount;
 	unsigned int order;
 	struct hyp_pool *pool;
-	struct list_head node;
 };
 
 extern u64 __hyp_vmemmap;

arch/arm64/kvm/hyp/nvhe/page_alloc.c

@@ -62,6 +62,34 @@ static struct hyp_page *__find_buddy_avail(struct hyp_pool *pool,
 
 }
 
+/*
+ * Pages that are available for allocation are tracked in free-lists, so we use
+ * the pages themselves to store the list nodes to avoid wasting space. As the
+ * allocator always returns zeroed pages (which are zeroed on the hyp_put_page()
+ * path to optimize allocation speed), we also need to clean-up the list node in
+ * each page when we take it out of the list.
+ */
+static inline void page_remove_from_list(struct hyp_page *p)
+{
+	struct list_head *node = hyp_page_to_virt(p);
+
+	__list_del_entry(node);
+	memset(node, 0, sizeof(*node));
+}
+
+static inline void page_add_to_list(struct hyp_page *p, struct list_head *head)
+{
+	struct list_head *node = hyp_page_to_virt(p);
+
+	INIT_LIST_HEAD(node);
+	list_add_tail(node, head);
+}
+
+static inline struct hyp_page *node_to_page(struct list_head *node)
+{
+	return hyp_virt_to_page(node);
+}
+
 static void __hyp_attach_page(struct hyp_pool *pool,
 			      struct hyp_page *p)
 {
@@ -83,14 +111,14 @@ static void __hyp_attach_page(struct hyp_pool *pool,
 			break;
 
 		/* Take the buddy out of its list, and coallesce with @p */
-		list_del_init(&buddy->node);
+		page_remove_from_list(buddy);
 		buddy->order = HYP_NO_ORDER;
 		p = min(p, buddy);
 	}
 
 	/* Mark the new head, and insert it */
 	p->order = order;
-	list_add_tail(&p->node, &pool->free_area[order]);
+	page_add_to_list(p, &pool->free_area[order]);
 }
 
 static struct hyp_page *__hyp_extract_page(struct hyp_pool *pool,
@@ -99,7 +127,7 @@ static struct hyp_page *__hyp_extract_page(struct hyp_pool *pool,
 {
 	struct hyp_page *buddy;
 
-	list_del_init(&p->node);
+	page_remove_from_list(p);
 	while (p->order > order) {
 		/*
 		 * The buddy of order n - 1 currently has HYP_NO_ORDER as it
@@ -110,7 +138,7 @@ static struct hyp_page *__hyp_extract_page(struct hyp_pool *pool,
 		p->order--;
 		buddy = __find_buddy_nocheck(pool, p, p->order);
 		buddy->order = p->order;
-		list_add_tail(&buddy->node, &pool->free_area[buddy->order]);
+		page_add_to_list(buddy, &pool->free_area[buddy->order]);
 	}
 
 	return p;
@@ -182,7 +210,7 @@ void *hyp_alloc_pages(struct hyp_pool *pool, unsigned int order)
 	}
 
 	/* Extract it from the tree at the right order */
-	p = list_first_entry(&pool->free_area[i], struct hyp_page, node);
+	p = node_to_page(pool->free_area[i].next);
 	p = __hyp_extract_page(pool, p, order);
 
 	hyp_set_page_refcounted(p);
@@ -210,7 +238,6 @@ int hyp_pool_init(struct hyp_pool *pool, u64 pfn, unsigned int nr_pages,
 	for (i = 0; i < nr_pages; i++) {
 		p[i].pool = pool;
 		p[i].order = 0;
-		INIT_LIST_HEAD(&p[i].node);
 		hyp_set_page_refcounted(&p[i]);
 	}