xdp: save a bit memory (in CPU caches)

src/libknot/xdp/af_xdp.c

@@ -54,7 +54,7 @@ _Static_assert((FRAME_SIZE == 4096 || FRAME_SIZE == 2048)
 	 * our implementation assume that the rings can never get filled. */
 	&& IS_POWER_OF_2(UMEM_RING_LEN_RX) && UMEM_RING_LEN_RX > UMEM_FRAME_COUNT_RX
 	&& IS_POWER_OF_2(UMEM_RING_LEN_TX) && UMEM_RING_LEN_TX > UMEM_FRAME_COUNT_TX
-	&& true
+	&& UMEM_FRAME_COUNT_TX <= (1 << 16) /* see tx_free_indices */
 	, "Incorrect #define combination for AF_XDP.");
 #endif
 

src/libknot/xdp/bpf-user.h

@@ -71,7 +71,7 @@ struct xsk_umem_info {
 
 	struct umem_frame *frames; /**< The memory frames. TODO: (uint8_t *frammem) might be more practical. */
 	uint32_t tx_free_count; /**< The number of free frames (for TX). */
-	uint32_t tx_free_indices[]; /**< Stack of indices of the free frames (for TX). */
+	uint16_t tx_free_indices[]; /**< Stack of indices of the free frames (for TX). */
 };
 
 typedef struct knot_xsk_socket {