iommu: convert mutex to rwlock for @map->lock

The mapping table managed for IOMMU page table is a skiplist protected
by mutex with @map->lock. This has led performance impact with multiple
threads looking up the page table at the same time, especially when
manipulating PRP or SGL data structures per I/O.

Application might optimize their own IOMMU-related paths like they
prepare the bulk of memory buffers at the beginning time and map them to
IOMMU at that time.  After that, they can do I/Os based on pre-mapped
memory buffers without inserting or deleting entries from page tables.

To improve performance scalability, convert existing mutex @map->lock to
read-write lock which leads to great performance on read(look-up)-most
scenarios.

Signed-off-by: Minwoo Im <[email protected]>

src/iommu/context.c

@@ -84,5 +84,5 @@ void iommu_ctx_init(struct iommu_ctx *ctx)
 	pthread_mutex_init(&ctx->lock, NULL);
 
 	skiplist_init(&ctx->map.list);
-	pthread_mutex_init(&ctx->map.lock, NULL);
+	pthread_rwlock_init(&ctx->map.lock, NULL);
 }

src/iommu/context.h

@@ -40,7 +40,7 @@ struct iova_mapping {
 };
 
 struct iova_map {
-	pthread_mutex_t lock;
+	pthread_rwlock_t lock;
 	struct skiplist list;
 };
 

src/iommu/dma.c

@@ -41,7 +41,7 @@ static int iova_cmp(const void *vaddr, const struct skiplist_node *n)
 static int iova_map_add(struct iova_map *map, void *vaddr, size_t len, uint64_t iova,
 			unsigned long flags)
 {
-	__autolock(&map->lock);
+	__autowrlock(&map->lock);
 
 	struct skiplist_node *update[SKIPLIST_LEVELS] = {};
 	struct iova_mapping *m;
@@ -70,7 +70,7 @@ static int iova_map_add(struct iova_map *map, void *vaddr, size_t len, uint64_t
 
 static void iova_map_remove(struct iova_map *map, void *vaddr)
 {
-	__autolock(&map->lock);
+	__autowrlock(&map->lock);
 
 	struct skiplist_node *n, *update[SKIPLIST_LEVELS] = {};
 
@@ -83,15 +83,15 @@ static void iova_map_remove(struct iova_map *map, void *vaddr)
 
 static struct iova_mapping *iova_map_find(struct iova_map *map, void *vaddr)
 {
-	__autolock(&map->lock);
+	__autordlock(&map->lock);
 
 	return container_of_or_null(skiplist_find(&map->list, vaddr, iova_cmp, NULL),
 				    struct iova_mapping, list);
 }
 
 static void iova_map_clear_with(struct iova_map *map, skiplist_iter_fn fn, void *opaque)
 {
-	__autolock(&map->lock);
+	__autowrlock(&map->lock);
 
 	skiplist_clear_with(&map->list, fn, opaque);
 }