Re: [dpdk-dev] [PATCH] mbuf: align rte_mbuf for Windows

19/05/2020 21:57, Dmitry Kozlyuk:

On Tue, 19 May 2020 20:49:50 +0200
Thomas Monjalon [off-list ref] wrote:

quoted

+Cc more maintainers

19/05/2020 20:41, talshn@mellanox.com:

quoted

From: Tal Shnaiderman <redacted>

Using uint32_t type bit-fields in Windows will pads the
'L2/L3/L4 and tunnel information' union with additional bits.

This padding causes rte_mbuf size misalignment and the total size
increases to 3 cache-lines.

Changed packet_type bit-fields types from uint32_t to uint8_t
to allow unified 2 cache-line structure size.

Added the __extension__ attribute over the modified struct to avoid
the warning:

type of bit-field ... is a GCC extension [-pedantic]

Signed-off-by: Tal Shnaiderman <redacted>
---
 lib/librte_mbuf/rte_mbuf_core.h | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/lib/librte_mbuf/rte_mbuf_core.h

b/lib/librte_mbuf/rte_mbuf_core.h index b9a59c879..82441555e 100644

--- a/lib/librte_mbuf/rte_mbuf_core.h
+++ b/lib/librte_mbuf/rte_mbuf_core.h

@@ -521,11 +521,12 @@ struct rte_mbuf {
 	RTE_STD_C11
 	union {
 		uint32_t packet_type; /**< L2/L3/L4 and tunnel

information. */
+		__extension__
 		struct {
-			uint32_t l2_type:4; /**< (Outer) L2 type.
*/
-			uint32_t l3_type:4; /**< (Outer) L3 type.
*/
-			uint32_t l4_type:4; /**< (Outer) L4 type.
*/
-			uint32_t tun_type:4; /**< Tunnel type. */
+			uint8_t l2_type:4; /**< (Outer) L2 type. */
+			uint8_t l3_type:4; /**< (Outer) L3 type. */
+			uint8_t l4_type:4; /**< (Outer) L4 type. */
+			uint8_t tun_type:4; /**< Tunnel type. */
 			RTE_STD_C11
 			union {
 				uint8_t inner_esp_next_proto;

@@ -541,7 +542,7 @@ struct rte_mbuf {
 					/**< Inner L3 type. */
 				};
 			};
-			uint32_t inner_l4_type:4; /**< Inner L4

type. */
+			uint8_t inner_l4_type:4; /**< Inner L4
type. */ };
 	};  

Such a clean and simple solution to what seemed to require compiler
workaround or fix! All offsets are equal on Windows and Linux for the
following toolchains, x86_64:

* cross-compilation with MinGW-w64 6.0.0 GCC 9.3.0
* Windows native MinGW-w64 6.0.0 GCC 8.1.0 and Clang 9.0.1

Tested-by: Dmitry Kozlyuk <redacted>

Would be interesting to see an offset comparison in little and big endian.