[dpdk-dev] [PATCH v2 4/5] mbuf: add userdata pointer field

[Bruce Richardson <bruce.richardson@intel.com>]

While some applications may store metadata about packets in the packet
mbuf headroom, this is not a workable solution for packet metadata which
is either:
* larger than the headroom (or headroom is needed for adding pkt headers)
* needs to be shared or copied among packets

To support these use cases in applications, we reserve a general
"userdata" pointer field inside the second cache-line of the mbuf. This
is better than having the application store the pointer to the external
metadata in the packet headroom, as it saves an additional cache-line
from being used.

Apart from storing metadata, this field also provides a general 8-byte
scratch space inside the mbuf for any other application uses that are
applicable.

Changes in V2:
* made the userdata field always have 8-bytes available, even on 32-bit

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
 lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h | 6 ++++--
 lib/librte_mbuf/rte_mbuf.h                                    | 6 ++++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h b/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
index 25ed672..e548161 100644
--- a/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
+++ b/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
@@ -116,8 +116,10 @@ struct rte_kni_mbuf {
 	char pad2[2];
 	uint16_t data_len;      /**< Amount of data in segment buffer. */
 	uint32_t pkt_len;       /**< Total pkt len: sum of all segment data_len. */
-	char pad3[8];
-	void *pool __attribute__((__aligned__(64)));
+
+	/* fields on second cache line */
+	char pad3[8] __attribute__((__aligned__(64)));
+	void *pool;
 	void *next;
 };
 
diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index 8e27d2e..9e70d3b 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -172,6 +172,12 @@ struct rte_mbuf {
 
 	/* second cache line - fields only used in slow path or on TX */
 	MARKER cacheline1 __rte_cache_aligned;
+
+	union {
+		void *userdata;   /**< Can be used for external metadata */
+		uint64_t udata64; /**< Allow 8-byte userdata on 32-bit */
+	};
+
 	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
 	struct rte_mbuf *next;    /**< Next segment of scattered packet. */
 
-- 
1.9.3