swiotlb: reduce swiotlb pool lookups

With CONFIG_SWIOTLB_DYNAMIC enabled, each round-trip map/unmap pair
in the swiotlb results in 6 calls to swiotlb_find_pool(). In multiple
places, the pool is found and used in one function, and then must
be found again in the next function that is called because only the
tlb_addr is passed as an argument. These are the six call sites:

dma_direct_map_page:
 1. swiotlb_map -> swiotlb_tbl_map_single -> swiotlb_bounce

dma_direct_unmap_page:
 2. dma_direct_sync_single_for_cpu -> is_swiotlb_buffer
 3. dma_direct_sync_single_for_cpu -> swiotlb_sync_single_for_cpu ->
	swiotlb_bounce
 4. is_swiotlb_buffer
 5. swiotlb_tbl_unmap_single -> swiotlb_del_transient
 6. swiotlb_tbl_unmap_single -> swiotlb_release_slots

Reduce the number of calls by finding the pool at a higher level, and
passing it as an argument instead of searching again. A key change is
for is_swiotlb_buffer() to return a pool pointer instead of a boolean,
and then pass this pool pointer to subsequent swiotlb functions.

There are 9 occurrences of is_swiotlb_buffer() used to test if a buffer
is a swiotlb buffer before calling a swiotlb function. To reduce code
duplication in getting the pool pointer and passing it as an argument,
introduce inline wrappers for this pattern. The generated code is
essentially unchanged.

Since is_swiotlb_buffer() no longer returns a boolean, rename some
functions to reflect the change:

 * swiotlb_find_pool() becomes __swiotlb_find_pool()
 * is_swiotlb_buffer() becomes swiotlb_find_pool()
 * is_xen_swiotlb_buffer() becomes xen_swiotlb_find_pool()

With these changes, a round-trip map/unmap pair requires only 2 pool
lookups (listed using the new names and wrappers):

dma_direct_unmap_page:
 1. dma_direct_sync_single_for_cpu -> swiotlb_find_pool
 2. swiotlb_tbl_unmap_single -> swiotlb_find_pool

These changes come from noticing the inefficiencies in a code review,
not from performance measurements. With CONFIG_SWIOTLB_DYNAMIC,
__swiotlb_find_pool() is not trivial, and it uses an RCU read lock,
so avoiding the redundant calls helps performance in a hot path.
When CONFIG_SWIOTLB_DYNAMIC is *not* set, the code size reduction
is minimal and the perf benefits are likely negligible, but no
harm is done.

No functional change is intended.

Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Petr Tesarik <petr@tesarici.cz>
Signed-off-by: Christoph Hellwig <hch@lst.de>

1 files changed, 62 insertions, 43 deletions

diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index 14bc10c1bb23..3dae0f592063 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -42,24 +42,6 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask,
 	int (*remap)(void *tlb, unsigned long nslabs));
 extern void __init swiotlb_update_mem_attributes(void);
 
-phys_addr_t swiotlb_tbl_map_single(struct device *hwdev, phys_addr_t phys,
-		size_t mapping_size,
-		unsigned int alloc_aligned_mask, enum dma_data_direction dir,
-		unsigned long attrs);
-
-extern void swiotlb_tbl_unmap_single(struct device *hwdev,
-				     phys_addr_t tlb_addr,
-				     size_t mapping_size,
-				     enum dma_data_direction dir,
-				     unsigned long attrs);
-
-void swiotlb_sync_single_for_device(struct device *dev, phys_addr_t tlb_addr,
-		size_t size, enum dma_data_direction dir);
-void swiotlb_sync_single_for_cpu(struct device *dev, phys_addr_t tlb_addr,
-		size_t size, enum dma_data_direction dir);
-dma_addr_t swiotlb_map(struct device *dev, phys_addr_t phys,
-		size_t size, enum dma_data_direction dir, unsigned long attrs);
-
 #ifdef CONFIG_SWIOTLB
 
 /**
@@ -143,37 +125,27 @@ struct io_tlb_mem {
 #endif
 };
 
-#ifdef CONFIG_SWIOTLB_DYNAMIC
-
-struct io_tlb_pool *swiotlb_find_pool(struct device *dev, phys_addr_t paddr);
-
-#else
-
-static inline struct io_tlb_pool *swiotlb_find_pool(struct device *dev,
-						    phys_addr_t paddr)
-{
-	return &dev->dma_io_tlb_mem->defpool;
-}
-
-#endif
+struct io_tlb_pool *__swiotlb_find_pool(struct device *dev, phys_addr_t paddr);
 
 /**
- * is_swiotlb_buffer() - check if a physical address belongs to a swiotlb
+ * swiotlb_find_pool() - find swiotlb pool to which a physical address belongs
  * @dev:        Device which has mapped the buffer.
  * @paddr:      Physical address within the DMA buffer.
  *
- * Check if @paddr points into a bounce buffer.
+ * Find the swiotlb pool that @paddr points into.
  *
  * Return:
- * * %true if @paddr points into a bounce buffer
- * * %false otherwise
+ * * pool address if @paddr points into a bounce buffer
+ * * NULL if @paddr does not point into a bounce buffer. As such, this function
+ *   can be used to determine if @paddr denotes a swiotlb bounce buffer.
  */
-static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
+static inline struct io_tlb_pool *swiotlb_find_pool(struct device *dev,
+		phys_addr_t paddr)
 {
 	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
 
 	if (!mem)
-		return false;
+		return NULL;
 
 #ifdef CONFIG_SWIOTLB_DYNAMIC
 	/*
@@ -182,16 +154,19 @@ static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
 	 * If a SWIOTLB address is checked on another CPU, then it was
 	 * presumably loaded by the device driver from an unspecified private
 	 * data structure. Make sure that this load is ordered before reading
-	 * dev->dma_uses_io_tlb here and mem->pools in swiotlb_find_pool().
+	 * dev->dma_uses_io_tlb here and mem->pools in __swiotlb_find_pool().
 	 *
 	 * This barrier pairs with smp_mb() in swiotlb_find_slots().
 	 */
 	smp_rmb();
-	return READ_ONCE(dev->dma_uses_io_tlb) &&
-		swiotlb_find_pool(dev, paddr);
+	if (READ_ONCE(dev->dma_uses_io_tlb))
+		return __swiotlb_find_pool(dev, paddr);
 #else
-	return paddr >= mem->defpool.start && paddr < mem->defpool.end;
+	if (paddr >= mem->defpool.start && paddr < mem->defpool.end)
+		return &mem->defpool;
 #endif
+
+	return NULL;
 }
 
 static inline bool is_swiotlb_force_bounce(struct device *dev)
@@ -219,9 +194,10 @@ static inline void swiotlb_dev_init(struct device *dev)
 {
 }
 
-static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
+static inline struct io_tlb_pool *swiotlb_find_pool(struct device *dev,
+		phys_addr_t paddr)
 {
-	return false;
+	return NULL;
 }
 static inline bool is_swiotlb_force_bounce(struct device *dev)
 {
@@ -260,6 +236,49 @@ static inline phys_addr_t default_swiotlb_limit(void)
 }
 #endif /* CONFIG_SWIOTLB */
 
+phys_addr_t swiotlb_tbl_map_single(struct device *hwdev, phys_addr_t phys,
+		size_t mapping_size, unsigned int alloc_aligned_mask,
+		enum dma_data_direction dir, unsigned long attrs);
+dma_addr_t swiotlb_map(struct device *dev, phys_addr_t phys,
+		size_t size, enum dma_data_direction dir, unsigned long attrs);
+
+void __swiotlb_tbl_unmap_single(struct device *hwdev, phys_addr_t tlb_addr,
+		size_t mapping_size, enum dma_data_direction dir,
+		unsigned long attrs, struct io_tlb_pool *pool);
+static inline void swiotlb_tbl_unmap_single(struct device *dev,
+		phys_addr_t addr, size_t size, enum dma_data_direction dir,
+		unsigned long attrs)
+{
+	struct io_tlb_pool *pool = swiotlb_find_pool(dev, addr);
+
+	if (unlikely(pool))
+		__swiotlb_tbl_unmap_single(dev, addr, size, dir, attrs, pool);
+}
+
+void __swiotlb_sync_single_for_device(struct device *dev, phys_addr_t tlb_addr,
+		size_t size, enum dma_data_direction dir,
+		struct io_tlb_pool *pool);
+static inline void swiotlb_sync_single_for_device(struct device *dev,
+		phys_addr_t addr, size_t size, enum dma_data_direction dir)
+{
+	struct io_tlb_pool *pool = swiotlb_find_pool(dev, addr);
+
+	if (unlikely(pool))
+		__swiotlb_sync_single_for_device(dev, addr, size, dir, pool);
+}
+
+void __swiotlb_sync_single_for_cpu(struct device *dev, phys_addr_t tlb_addr,
+		size_t size, enum dma_data_direction dir,
+		struct io_tlb_pool *pool);
+static inline void swiotlb_sync_single_for_cpu(struct device *dev,
+		phys_addr_t addr, size_t size, enum dma_data_direction dir)
+{
+	struct io_tlb_pool *pool = swiotlb_find_pool(dev, addr);
+
+	if (unlikely(pool))
+		__swiotlb_sync_single_for_cpu(dev, addr, size, dir, pool);
+}
+
 extern void swiotlb_print_info(void);
 
 #ifdef CONFIG_DMA_RESTRICTED_POOL