[dpdk-dev] [PATCH v2 1/2] mbuf: add rte_pktmbuff_reset_headroom function

[Maxime Coquelin]

Some application use rte_mbuf_raw_alloc() function to improve
performance by not resetting mbuf's fields to their default state.

This can be however problematic for mbuf consumers that need some
headroom, meaning that data_off field gets decremented after
allocation. When the mbuf is re-used afterwards, there might not
be enough room for the consumer to prepend anything, if the data_off
field is not reset to its default value.

This patch adds a new rte_pktmbuf_reset_headroom() function that
applications can call to reset the data_off field.
This patch also replaces current data_off affectations in the mbuf
lib with a call to this function.

Signed-off-by: Maxime Coquelin <maxime.coquelin at redhat.com>
---
Changes since v2:
=================
 - Specify headroom may be reset only if segment is empty.

 lib/librte_mbuf/rte_mbuf.h | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index 23b7bf8..85a653c 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -1387,6 +1387,19 @@ rte_pktmbuf_priv_size(struct rte_mempool *mp)
 }
 
 /**
+ * Reset the data_off field of a packet mbuf to its default value.
+ *
+ * The given mbuf must have only one segment, which should be empty.
+ *
+ * @param m
+ *   The packet mbuf's data_off field has to be reset.
+ */
+static inline void rte_pktmbuf_reset_headroom(struct rte_mbuf *m)
+{
+	m->data_off = RTE_MIN(RTE_PKTMBUF_HEADROOM, (uint16_t)m->buf_len);
+}
+
+/**
  * Reset the fields of a packet mbuf to their default values.
  *
  * The given mbuf must have only one segment.
@@ -1406,8 +1419,7 @@ static inline void rte_pktmbuf_reset(struct rte_mbuf *m)
 
 	m->ol_flags = 0;
 	m->packet_type = 0;
-	m->data_off = (RTE_PKTMBUF_HEADROOM <= m->buf_len) ?
-			RTE_PKTMBUF_HEADROOM : m->buf_len;
+	rte_pktmbuf_reset_headroom(m);
 
 	m->data_len = 0;
 	__rte_mbuf_sanity_check(m, 1);
@@ -1571,7 +1583,7 @@ static inline void rte_pktmbuf_detach(struct rte_mbuf *m)
 	m->buf_addr = (char *)m + mbuf_size;
 	m->buf_physaddr = rte_mempool_virt2phy(mp, m) + mbuf_size;
 	m->buf_len = (uint16_t)buf_len;
-	m->data_off = RTE_MIN(RTE_PKTMBUF_HEADROOM, (uint16_t)m->buf_len);
+	rte_pktmbuf_reset_headroom(m);
 	m->data_len = 0;
 	m->ol_flags = 0;
 
-- 
2.7.4