xsk: Use pool->dma_pages to check for DMA

Compare pool->dma_pages instead of pool->dma_pages_cnt to check for an
active DMA mapping. pool->dma_pages needs to be read anyway to access
the map so this compiles to more efficient code.
Signed-off-by: Kal Conley <kal.conley@dectris.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20230423180157.93559-1-kal.conley@dectris.com

include/net/xsk_buff_pool.h

@@ -180,7 +180,7 @@ static inline bool xp_desc_crosses_non_contig_pg(struct xsk_buff_pool *pool,
 	if (likely(!cross_pg))
 		return false;
 
-	return pool->dma_pages_cnt &&
+	return pool->dma_pages &&
 	       !(pool->dma_pages[addr >> PAGE_SHIFT] & XSK_NEXT_PG_CONTIG_MASK);
 }
 

net/xdp/xsk_buff_pool.c

@@ -350,7 +350,7 @@ void xp_dma_unmap(struct xsk_buff_pool *pool, unsigned long attrs)
 {
 	struct xsk_dma_map *dma_map;
 
-	if (pool->dma_pages_cnt == 0)
+	if (!pool->dma_pages)
 		return;
 
 	dma_map = xp_find_dma_map(pool);
@@ -364,6 +364,7 @@ void xp_dma_unmap(struct xsk_buff_pool *pool, unsigned long attrs)
 
 	__xp_dma_unmap(dma_map, attrs);
 	kvfree(pool->dma_pages);
+	pool->dma_pages = NULL;
 	pool->dma_pages_cnt = 0;
 	pool->dev = NULL;
 }
@@ -503,7 +504,7 @@ static struct xdp_buff_xsk *__xp_alloc(struct xsk_buff_pool *pool)
 	if (pool->unaligned) {
 		xskb = pool->free_heads[--pool->free_heads_cnt];
 		xp_init_xskb_addr(xskb, pool, addr);
-		if (pool->dma_pages_cnt)
+		if (pool->dma_pages)
 			xp_init_xskb_dma(xskb, pool, pool->dma_pages, addr);
 	} else {
 		xskb = &pool->heads[xp_aligned_extract_idx(pool, addr)];
@@ -569,7 +570,7 @@ static u32 xp_alloc_new_from_fq(struct xsk_buff_pool *pool, struct xdp_buff **xd
 		if (pool->unaligned) {
 			xskb = pool->free_heads[--pool->free_heads_cnt];
 			xp_init_xskb_addr(xskb, pool, addr);
-			if (pool->dma_pages_cnt)
+			if (pool->dma_pages)
 				xp_init_xskb_dma(xskb, pool, pool->dma_pages, addr);
 		} else {
 			xskb = &pool->heads[xp_aligned_extract_idx(pool, addr)];