KVM: arm64: Use refcount at hyp to check page availability

The hyp buddy allocator currently checks the struct hyp_page list node
to see if a page is available for allocation or not when trying to
coalesce memory. Now that decrementing the refcount and attaching to
the buddy tree is done in the same critical section, we can rely on the
refcount of the buddy page to be in sync, which allows to replace the
list node check by a refcount check. This will ease removing the list
node from struct hyp_page later on.

Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210608114518.748712-3-qperret@google.com

Author: Quentin Perret

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

@@ -55,7 +55,7 @@ static struct hyp_page *__find_buddy_avail(struct hyp_pool *pool,
 {
 	struct hyp_page *buddy = __find_buddy_nocheck(pool, p, order);
 
-	if (!buddy || buddy->order != order || list_empty(&buddy->node))
+	if (!buddy || buddy->order != order || buddy->refcount)
 		return NULL;
 
 	return buddy;
@@ -133,6 +133,12 @@ static inline void hyp_set_page_refcounted(struct hyp_page *p)
 	p->refcount = 1;
 }
 
+static void __hyp_put_page(struct hyp_pool *pool, struct hyp_page *p)
+{
+	if (hyp_page_ref_dec_and_test(p))
+		__hyp_attach_page(pool, p);
+}
+
 /*
  * Changes to the buddy tree and page refcounts must be done with the hyp_pool
  * lock held. If a refcount change requires an update to the buddy tree (e.g.
@@ -146,8 +152,7 @@ void hyp_put_page(void *addr)
 	struct hyp_pool *pool = hyp_page_to_pool(p);
 
 	hyp_spin_lock(&pool->lock);
-	if (hyp_page_ref_dec_and_test(p))
-		__hyp_attach_page(pool, p);
+	__hyp_put_page(pool, p);
 	hyp_spin_unlock(&pool->lock);
 }
 
@@ -202,15 +207,16 @@ int hyp_pool_init(struct hyp_pool *pool, u64 pfn, unsigned int nr_pages,
 
 	/* Init the vmemmap portion */
 	p = hyp_phys_to_page(phys);
-	memset(p, 0, sizeof(*p) * 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]);
 	}
 
 	/* Attach the unused pages to the buddy tree */
 	for (i = reserved_pages; i < nr_pages; i++)
-		__hyp_attach_page(pool, &p[i]);
+		__hyp_put_page(pool, &p[i]);
 
 	return 0;
 }