[dpdk-dev] [PATCH v3] mbuf: support dynamic fields and flags

[Olivier Matz <olivier.matz@6wind.com>]

Many features require to store data inside the mbuf. As the room in mbuf
structure is limited, it is not possible to have a field for each
feature. Also, changing fields in the mbuf structure can break the API
or ABI.

This commit addresses these issues, by enabling the dynamic registration
of fields or flags:

- a dynamic field is a named area in the rte_mbuf structure, with a
  given size (>= 1 byte) and alignment constraint.
- a dynamic flag is a named bit in the rte_mbuf structure.

The typical use case is a PMD that registers space for an offload
feature, when the application requests to enable this feature.  As
the space in mbuf is limited, the space should only be reserved if it
is going to be used (i.e when the application explicitly asks for it).

The registration can be done at any moment, but it is not possible
to unregister fields or flags.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
---

v3

* define mark_free() macro outside the init_shared_mem() function
  (Konstantin)
* better document automatic field placement (Konstantin)
* introduce RTE_SIZEOF_FIELD() to get the size of a field in
  a structure (Haiyue)
* fix api doc generation (Slava)
* document dynamic field and flags naming conventions

v2

* Rebase on top of master: solve conflict with Stephen's patchset
  (packet copy)
* Add new apis to register a dynamic field/flag at a specific place
* Add a dump function (sugg by David)
* Enhance field registration function to select the best offset, keeping
  large aligned zones as much as possible (sugg by Konstantin)
* Use a size_t and unsigned int instead of int when relevant
  (sugg by Konstantin)
* Use "uint64_t dynfield1[2]" in mbuf instead of 2 uint64_t fields
  (sugg by Konstantin)
* Remove unused argument in private function (sugg by Konstantin)
* Fix and simplify locking (sugg by Konstantin)
* Fix minor typo

rfc -> v1

* Rebase on top of master
* Change registration API to use a structure instead of
  variables, getting rid of #defines (Stephen's comment)
* Update flag registration to use a similar API as fields.
* Change max name length from 32 to 64 (sugg. by Thomas)
* Enhance API documentation (Haiyue's and Andrew's comments)
* Add a debug log at registration
* Add some words in release note
* Did some performance tests (sugg. by Andrew):
  On my platform, reading a dynamic field takes ~3 cycles more
  than a static field, and ~2 cycles more for writing.


 app/test/test_mbuf.c                       | 145 +++++-
 doc/guides/rel_notes/release_19_11.rst     |   7 +
 lib/librte_eal/common/include/rte_common.h |  12 +
 lib/librte_mbuf/Makefile                   |   2 +
 lib/librte_mbuf/meson.build                |   6 +-
 lib/librte_mbuf/rte_mbuf.h                 |  23 +-
 lib/librte_mbuf/rte_mbuf_dyn.c             | 553 +++++++++++++++++++++
 lib/librte_mbuf/rte_mbuf_dyn.h             | 239 +++++++++
 lib/librte_mbuf/rte_mbuf_version.map       |   7 +
 9 files changed, 989 insertions(+), 5 deletions(-)
 create mode 100644 lib/librte_mbuf/rte_mbuf_dyn.c
 create mode 100644 lib/librte_mbuf/rte_mbuf_dyn.h

diff --git a/app/test/test_mbuf.c b/app/test/test_mbuf.c
index c21ef64c8..e9be430af 100644
--- a/app/test/test_mbuf.c
+++ b/app/test/test_mbuf.c
@@ -29,6 +29,7 @@
 #include <rte_random.h>
 #include <rte_cycles.h>
 #include <rte_malloc.h>
+#include <rte_mbuf_dyn.h>
 
 #include "test.h"
 
@@ -661,7 +662,6 @@ test_attach_from_different_pool(struct rte_mempool *pktmbuf_pool,
 		rte_pktmbuf_free(clone2);
 	return -1;
 }
-#undef GOTO_FAIL
 
 /*
  * test allocation and free of mbufs
@@ -1449,6 +1449,143 @@ test_tx_offload(void)
 	return (v1 == v2) ? 0 : -EINVAL;
 }
 
+static int
+test_mbuf_dyn(struct rte_mempool *pktmbuf_pool)
+{
+	const struct rte_mbuf_dynfield dynfield = {
+		.name = "test-dynfield",
+		.size = sizeof(uint8_t),
+		.align = __alignof__(uint8_t),
+		.flags = 0,
+	};
+	const struct rte_mbuf_dynfield dynfield2 = {
+		.name = "test-dynfield2",
+		.size = sizeof(uint16_t),
+		.align = __alignof__(uint16_t),
+		.flags = 0,
+	};
+	const struct rte_mbuf_dynfield dynfield3 = {
+		.name = "test-dynfield3",
+		.size = sizeof(uint8_t),
+		.align = __alignof__(uint8_t),
+		.flags = 0,
+	};
+	const struct rte_mbuf_dynfield dynfield_fail_big = {
+		.name = "test-dynfield-fail-big",
+		.size = 256,
+		.align = 1,
+		.flags = 0,
+	};
+	const struct rte_mbuf_dynfield dynfield_fail_align = {
+		.name = "test-dynfield-fail-align",
+		.size = 1,
+		.align = 3,
+		.flags = 0,
+	};
+	const struct rte_mbuf_dynflag dynflag = {
+		.name = "test-dynflag",
+		.flags = 0,
+	};
+	const struct rte_mbuf_dynflag dynflag2 = {
+		.name = "test-dynflag2",
+		.flags = 0,
+	};
+	const struct rte_mbuf_dynflag dynflag3 = {
+		.name = "test-dynflag3",
+		.flags = 0,
+	};
+	struct rte_mbuf *m = NULL;
+	int offset, offset2, offset3;
+	int flag, flag2, flag3;
+	int ret;
+
+	printf("Test mbuf dynamic fields and flags\n");
+	rte_mbuf_dyn_dump(stdout);
+
+	offset = rte_mbuf_dynfield_register(&dynfield);
+	if (offset == -1)
+		GOTO_FAIL("failed to register dynamic field, offset=%d: %s",
+			offset, strerror(errno));
+
+	ret = rte_mbuf_dynfield_register(&dynfield);
+	if (ret != offset)
+		GOTO_FAIL("failed to lookup dynamic field, ret=%d: %s",
+			ret, strerror(errno));
+
+	offset2 = rte_mbuf_dynfield_register(&dynfield2);
+	if (offset2 == -1 || offset2 == offset || (offset2 & 1))
+		GOTO_FAIL("failed to register dynamic field 2, offset2=%d: %s",
+			offset2, strerror(errno));
+
+	offset3 = rte_mbuf_dynfield_register_offset(&dynfield3,
+				offsetof(struct rte_mbuf, dynfield1[1]));
+	if (offset3 != offsetof(struct rte_mbuf, dynfield1[1]))
+		GOTO_FAIL("failed to register dynamic field 3, offset=%d: %s",
+			offset3, strerror(errno));
+
+	printf("dynfield: offset=%d, offset2=%d, offset3=%d\n",
+		offset, offset2, offset3);
+
+	ret = rte_mbuf_dynfield_register(&dynfield_fail_big);
+	if (ret != -1)
+		GOTO_FAIL("dynamic field creation should fail (too big)");
+
+	ret = rte_mbuf_dynfield_register(&dynfield_fail_align);
+	if (ret != -1)
+		GOTO_FAIL("dynamic field creation should fail (bad alignment)");
+
+	ret = rte_mbuf_dynfield_register_offset(&dynfield_fail_align,
+				offsetof(struct rte_mbuf, ol_flags));
+	if (ret != -1)
+		GOTO_FAIL("dynamic field creation should fail (not avail)");
+
+	flag = rte_mbuf_dynflag_register(&dynflag);
+	if (flag == -1)
+		GOTO_FAIL("failed to register dynamic flag, flag=%d: %s",
+			flag, strerror(errno));
+
+	ret = rte_mbuf_dynflag_register(&dynflag);
+	if (ret != flag)
+		GOTO_FAIL("failed to lookup dynamic flag, ret=%d: %s",
+			ret, strerror(errno));
+
+	flag2 = rte_mbuf_dynflag_register(&dynflag2);
+	if (flag2 == -1 || flag2 == flag)
+		GOTO_FAIL("failed to register dynamic flag 2, flag2=%d: %s",
+			flag2, strerror(errno));
+
+	flag3 = rte_mbuf_dynflag_register_bitnum(&dynflag3,
+						rte_bsf64(PKT_LAST_FREE));
+	if (flag3 != rte_bsf64(PKT_LAST_FREE))
+		GOTO_FAIL("failed to register dynamic flag 3, flag2=%d: %s",
+			flag3, strerror(errno));
+
+	printf("dynflag: flag=%d, flag2=%d, flag3=%d\n", flag, flag2, flag3);
+
+	/* set, get dynamic field */
+	m = rte_pktmbuf_alloc(pktmbuf_pool);
+	if (m == NULL)
+		GOTO_FAIL("Cannot allocate mbuf");
+
+	*RTE_MBUF_DYNFIELD(m, offset, uint8_t *) = 1;
+	if (*RTE_MBUF_DYNFIELD(m, offset, uint8_t *) != 1)
+		GOTO_FAIL("failed to read dynamic field");
+	*RTE_MBUF_DYNFIELD(m, offset2, uint16_t *) = 1000;
+	if (*RTE_MBUF_DYNFIELD(m, offset2, uint16_t *) != 1000)
+		GOTO_FAIL("failed to read dynamic field");
+
+	/* set a dynamic flag */
+	m->ol_flags |= (1ULL << flag);
+
+	rte_mbuf_dyn_dump(stdout);
+	rte_pktmbuf_free(m);
+	return 0;
+fail:
+	rte_pktmbuf_free(m);
+	return -1;
+}
+#undef GOTO_FAIL
+
 static int
 test_mbuf(void)
 {
@@ -1468,6 +1605,12 @@ test_mbuf(void)
 		goto err;
 	}
 
+	/* test registration of dynamic fields and flags */
+	if (test_mbuf_dyn(pktmbuf_pool) < 0) {
+		printf("mbuf dynflag test failed\n");
+		goto err;
+	}
+
 	/* create a specific pktmbuf pool with a priv_size != 0 and no data
 	 * room size */
 	pktmbuf_pool2 = rte_pktmbuf_pool_create("test_pktmbuf_pool2",
diff --git a/doc/guides/rel_notes/release_19_11.rst b/doc/guides/rel_notes/release_19_11.rst
index 856088c5c..b7511a6dc 100644
--- a/doc/guides/rel_notes/release_19_11.rst
+++ b/doc/guides/rel_notes/release_19_11.rst
@@ -21,6 +21,13 @@ DPDK Release 19.11
 
       xdg-open build/doc/html/guides/rel_notes/release_19_11.html
 
+* **Add support of support dynamic fields and flags in mbuf.**
+
+  This new feature adds the ability to dynamically register some room
+  for a field or a flag in the mbuf structure. This is typically used
+  for specific offload features, where adding a static field or flag
+  in the mbuf is not justified.
+
 
 New Features
 ------------
diff --git a/lib/librte_eal/common/include/rte_common.h b/lib/librte_eal/common/include/rte_common.h
index 05a3a6401..6660c77e4 100644
--- a/lib/librte_eal/common/include/rte_common.h
+++ b/lib/librte_eal/common/include/rte_common.h
@@ -630,6 +630,18 @@ rte_log2_u64(uint64_t v)
 		})
 #endif
 
+/**
+ * Get the size of a field in a structure.
+ *
+ * @param type
+ *   The type of the structure.
+ * @param field
+ *   The field in the structure.
+ * @return
+ *   The size of the field in the structure, in bytes.
+ */
+#define RTE_SIZEOF_FIELD(type, field) (sizeof(((type *)0)->field))
+
 #define _RTE_STR(x) #x
 /** Take a macro value and get a string version of it */
 #define RTE_STR(x) _RTE_STR(x)
diff --git a/lib/librte_mbuf/Makefile b/lib/librte_mbuf/Makefile
index c8f6d2689..5a9bcee73 100644
--- a/lib/librte_mbuf/Makefile
+++ b/lib/librte_mbuf/Makefile
@@ -17,8 +17,10 @@ LIBABIVER := 5
 
 # all source are stored in SRCS-y
 SRCS-$(CONFIG_RTE_LIBRTE_MBUF) := rte_mbuf.c rte_mbuf_ptype.c rte_mbuf_pool_ops.c
+SRCS-$(CONFIG_RTE_LIBRTE_MBUF) += rte_mbuf_dyn.c
 
 # install includes
 SYMLINK-$(CONFIG_RTE_LIBRTE_MBUF)-include := rte_mbuf.h rte_mbuf_ptype.h rte_mbuf_pool_ops.h
+SYMLINK-$(CONFIG_RTE_LIBRTE_MBUF)-include += rte_mbuf_dyn.h
 
 include $(RTE_SDK)/mk/rte.lib.mk
diff --git a/lib/librte_mbuf/meson.build b/lib/librte_mbuf/meson.build
index 6cc11ebb4..9137e8f26 100644
--- a/lib/librte_mbuf/meson.build
+++ b/lib/librte_mbuf/meson.build
@@ -2,8 +2,10 @@
 # Copyright(c) 2017 Intel Corporation
 
 version = 5
-sources = files('rte_mbuf.c', 'rte_mbuf_ptype.c', 'rte_mbuf_pool_ops.c')
-headers = files('rte_mbuf.h', 'rte_mbuf_ptype.h', 'rte_mbuf_pool_ops.h')
+sources = files('rte_mbuf.c', 'rte_mbuf_ptype.c', 'rte_mbuf_pool_ops.c',
+	'rte_mbuf_dyn.c')
+headers = files('rte_mbuf.h', 'rte_mbuf_ptype.h', 'rte_mbuf_pool_ops.h',
+	'rte_mbuf_dyn.h')
 deps += ['mempool']
 
 allow_experimental_apis = true
diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index b1a92b17a..7567b6ff3 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -198,9 +198,12 @@ extern "C" {
 #define PKT_RX_OUTER_L4_CKSUM_GOOD	(1ULL << 22)
 #define PKT_RX_OUTER_L4_CKSUM_INVALID	((1ULL << 21) | (1ULL << 22))
 
-/* add new RX flags here */
+/* add new RX flags here, don't forget to update PKT_FIRST_FREE */
 
-/* add new TX flags here */
+#define PKT_FIRST_FREE (1ULL << 23)
+#define PKT_LAST_FREE (1ULL << 39)
+
+/* add new TX flags here, don't forget to update PKT_LAST_FREE  */
 
 /**
  * Indicate that the metadata field in the mbuf is in use.
@@ -738,6 +741,7 @@ struct rte_mbuf {
 	 */
 	struct rte_mbuf_ext_shared_info *shinfo;
 
+	uint64_t dynfield1[2]; /**< Reserved for dynamic fields. */
 } __rte_cache_aligned;
 
 /**
@@ -1684,6 +1688,20 @@ rte_pktmbuf_attach_extbuf(struct rte_mbuf *m, void *buf_addr,
  */
 #define rte_pktmbuf_detach_extbuf(m) rte_pktmbuf_detach(m)
 
+/**
+ * Copy dynamic fields from msrc to mdst.
+ *
+ * @param mdst
+ *   The destination mbuf.
+ * @param msrc
+ *   The source mbuf.
+ */
+static inline void
+rte_mbuf_dynfield_copy(struct rte_mbuf *mdst, const struct rte_mbuf *msrc)
+{
+	memcpy(&mdst->dynfield1, msrc->dynfield1, sizeof(mdst->dynfield1));
+}
+
 /* internal */
 static inline void
 __rte_pktmbuf_copy_hdr(struct rte_mbuf *mdst, const struct rte_mbuf *msrc)
@@ -1695,6 +1713,7 @@ __rte_pktmbuf_copy_hdr(struct rte_mbuf *mdst, const struct rte_mbuf *msrc)
 	mdst->hash = msrc->hash;
 	mdst->packet_type = msrc->packet_type;
 	mdst->timestamp = msrc->timestamp;
+	rte_mbuf_dynfield_copy(mdst, msrc);
 }
 
 /**
diff --git a/lib/librte_mbuf/rte_mbuf_dyn.c b/lib/librte_mbuf/rte_mbuf_dyn.c
new file mode 100644
index 000000000..d6931f847
--- /dev/null
+++ b/lib/librte_mbuf/rte_mbuf_dyn.c
@@ -0,0 +1,553 @@
+/* SPDX-License-Identifier: BSD-3-Clause
+ * Copyright 2019 6WIND S.A.
+ */
+
+#include <sys/queue.h>
+#include <stdint.h>
+#include <limits.h>
+
+#include <rte_common.h>
+#include <rte_eal.h>
+#include <rte_eal_memconfig.h>
+#include <rte_tailq.h>
+#include <rte_errno.h>
+#include <rte_malloc.h>
+#include <rte_string_fns.h>
+#include <rte_mbuf.h>
+#include <rte_mbuf_dyn.h>
+
+#define RTE_MBUF_DYN_MZNAME "rte_mbuf_dyn"
+
+struct mbuf_dynfield_elt {
+	TAILQ_ENTRY(mbuf_dynfield_elt) next;
+	struct rte_mbuf_dynfield params;
+	size_t offset;
+};
+TAILQ_HEAD(mbuf_dynfield_list, rte_tailq_entry);
+
+static struct rte_tailq_elem mbuf_dynfield_tailq = {
+	.name = "RTE_MBUF_DYNFIELD",
+};
+EAL_REGISTER_TAILQ(mbuf_dynfield_tailq);
+
+struct mbuf_dynflag_elt {
+	TAILQ_ENTRY(mbuf_dynflag_elt) next;
+	struct rte_mbuf_dynflag params;
+	unsigned int bitnum;
+};
+TAILQ_HEAD(mbuf_dynflag_list, rte_tailq_entry);
+
+static struct rte_tailq_elem mbuf_dynflag_tailq = {
+	.name = "RTE_MBUF_DYNFLAG",
+};
+EAL_REGISTER_TAILQ(mbuf_dynflag_tailq);
+
+struct mbuf_dyn_shm {
+	/**
+	 * For each mbuf byte, free_space[i] != 0 if space is free.
+	 * The value is the size of the biggest aligned element that
+	 * can fit in the zone.
+	 */
+	uint8_t free_space[sizeof(struct rte_mbuf)];
+	/** Bitfield of available flags. */
+	uint64_t free_flags;
+};
+static struct mbuf_dyn_shm *shm;
+
+/* Set the value of free_space[] according to the size and alignment of
+ * the free areas. This helps to select the best place when reserving a
+ * dynamic field. Assume tailq is locked.
+ */
+static void
+process_score(void)
+{
+	size_t off, align, size, i;
+
+	/* first, erase previous info */
+	for (i = 0; i < sizeof(struct rte_mbuf); i++) {
+		if (shm->free_space[i])
+			shm->free_space[i] = 1;
+	}
+
+	for (off = 0; off < sizeof(struct rte_mbuf); off++) {
+		/* get the size of the free zone */
+		for (size = 0; shm->free_space[off + size]; size++)
+			;
+		if (size == 0)
+			continue;
+
+		/* get the alignment of biggest object that can fit in
+		 * the zone at this offset.
+		 */
+		for (align = 1;
+		     (off % (align << 1)) == 0 && (align << 1) <= size;
+		     align <<= 1)
+			;
+
+		/* save it in free_space[] */
+		for (i = off; i < off + size; i++)
+			shm->free_space[i] = RTE_MAX(align, shm->free_space[i]);
+	}
+}
+
+/* Mark the area occupied by a mbuf field as available in the shm. */
+#define mark_free(field)						\
+	memset(&shm->free_space[offsetof(struct rte_mbuf, field)],	\
+		1, sizeof(((struct rte_mbuf *)0)->field))
+
+/* Allocate and initialize the shared memory. Assume tailq is locked */
+static int
+init_shared_mem(void)
+{
+	const struct rte_memzone *mz;
+	uint64_t mask;
+
+	if (rte_eal_process_type() == RTE_PROC_PRIMARY) {
+		mz = rte_memzone_reserve_aligned(RTE_MBUF_DYN_MZNAME,
+						sizeof(struct mbuf_dyn_shm),
+						SOCKET_ID_ANY, 0,
+						RTE_CACHE_LINE_SIZE);
+	} else {
+		mz = rte_memzone_lookup(RTE_MBUF_DYN_MZNAME);
+	}
+	if (mz == NULL)
+		return -1;
+
+	shm = mz->addr;
+
+	if (rte_eal_process_type() == RTE_PROC_PRIMARY) {
+		/* init free_space, keep it sync'd with
+		 * rte_mbuf_dynfield_copy().
+		 */
+		memset(shm, 0, sizeof(*shm));
+		mark_free(dynfield1);
+
+		/* init free_flags */
+		for (mask = PKT_FIRST_FREE; mask <= PKT_LAST_FREE; mask <<= 1)
+			shm->free_flags |= mask;
+
+		process_score();
+	}
+
+	return 0;
+}
+
+/* check if this offset can be used */
+static int
+check_offset(size_t offset, size_t size, size_t align)
+{
+	size_t i;
+
+	if ((offset & (align - 1)) != 0)
+		return -1;
+	if (offset + size > sizeof(struct rte_mbuf))
+		return -1;
+
+	for (i = 0; i < size; i++) {
+		if (!shm->free_space[i + offset])
+			return -1;
+	}
+
+	return 0;
+}
+
+/* assume tailq is locked */
+static struct mbuf_dynfield_elt *
+__mbuf_dynfield_lookup(const char *name)
+{
+	struct mbuf_dynfield_list *mbuf_dynfield_list;
+	struct mbuf_dynfield_elt *mbuf_dynfield;
+	struct rte_tailq_entry *te;
+
+	mbuf_dynfield_list = RTE_TAILQ_CAST(
+		mbuf_dynfield_tailq.head, mbuf_dynfield_list);
+
+	TAILQ_FOREACH(te, mbuf_dynfield_list, next) {
+		mbuf_dynfield = (struct mbuf_dynfield_elt *)te->data;
+		if (strcmp(name, mbuf_dynfield->params.name) == 0)
+			break;
+	}
+
+	if (te == NULL) {
+		rte_errno = ENOENT;
+		return NULL;
+	}
+
+	return mbuf_dynfield;
+}
+
+int
+rte_mbuf_dynfield_lookup(const char *name, struct rte_mbuf_dynfield *params)
+{
+	struct mbuf_dynfield_elt *mbuf_dynfield;
+
+	if (shm == NULL) {
+		rte_errno = ENOENT;
+		return -1;
+	}
+
+	rte_mcfg_tailq_read_lock();
+	mbuf_dynfield = __mbuf_dynfield_lookup(name);
+	rte_mcfg_tailq_read_unlock();
+
+	if (mbuf_dynfield == NULL) {
+		rte_errno = ENOENT;
+		return -1;
+	}
+
+	if (params != NULL)
+		memcpy(params, &mbuf_dynfield->params, sizeof(*params));
+
+	return mbuf_dynfield->offset;
+}
+
+static int mbuf_dynfield_cmp(const struct rte_mbuf_dynfield *params1,
+		const struct rte_mbuf_dynfield *params2)
+{
+	if (strcmp(params1->name, params2->name))
+		return -1;
+	if (params1->size != params2->size)
+		return -1;
+	if (params1->align != params2->align)
+		return -1;
+	if (params1->flags != params2->flags)
+		return -1;
+	return 0;
+}
+
+/* assume tailq is locked */
+static int
+__rte_mbuf_dynfield_register_offset(const struct rte_mbuf_dynfield *params,
+				size_t req)
+{
+	struct mbuf_dynfield_list *mbuf_dynfield_list;
+	struct mbuf_dynfield_elt *mbuf_dynfield = NULL;
+	struct rte_tailq_entry *te = NULL;
+	unsigned int best_zone = UINT_MAX;
+	size_t i, offset;
+	int ret;
+
+	if (shm == NULL && init_shared_mem() < 0)
+		return -1;
+
+	mbuf_dynfield = __mbuf_dynfield_lookup(params->name);
+	if (mbuf_dynfield != NULL) {
+		if (req != SIZE_MAX && req != mbuf_dynfield->offset) {
+			rte_errno = EEXIST;
+			return -1;
+		}
+		if (mbuf_dynfield_cmp(params, &mbuf_dynfield->params) < 0) {
+			rte_errno = EEXIST;
+			return -1;
+		}
+		return mbuf_dynfield->offset;
+	}
+
+	if (rte_eal_process_type() != RTE_PROC_PRIMARY) {
+		rte_errno = EPERM;
+		return -1;
+	}
+
+	if (req == SIZE_MAX) {
+		/* Find the best place to put this field: we search the
+		 * lowest value of shm->free_space[offset]: the zones
+		 * containing room for larger fields are kept for later.
+		 */
+		for (offset = 0;
+		     offset < sizeof(struct rte_mbuf);
+		     offset++) {
+			if (check_offset(offset, params->size,
+						params->align) == 0 &&
+					shm->free_space[offset] < best_zone) {
+				best_zone = shm->free_space[offset];
+				req = offset;
+			}
+		}
+		if (req == SIZE_MAX) {
+			rte_errno = ENOENT;
+			return -1;
+		}
+	} else {
+		if (check_offset(req, params->size, params->align) < 0) {
+			rte_errno = EBUSY;
+			return -1;
+		}
+	}
+
+	offset = req;
+	mbuf_dynfield_list = RTE_TAILQ_CAST(
+		mbuf_dynfield_tailq.head, mbuf_dynfield_list);
+
+	te = rte_zmalloc("MBUF_DYNFIELD_TAILQ_ENTRY", sizeof(*te), 0);
+	if (te == NULL)
+		return -1;
+
+	mbuf_dynfield = rte_zmalloc("mbuf_dynfield", sizeof(*mbuf_dynfield), 0);
+	if (mbuf_dynfield == NULL) {
+		rte_free(te);
+		return -1;
+	}
+
+	ret = strlcpy(mbuf_dynfield->params.name, params->name,
+		sizeof(mbuf_dynfield->params.name));
+	if (ret < 0 || ret >= (int)sizeof(mbuf_dynfield->params.name)) {
+		rte_errno = ENAMETOOLONG;
+		rte_free(mbuf_dynfield);
+		rte_free(te);
+		return -1;
+	}
+	memcpy(&mbuf_dynfield->params, params, sizeof(mbuf_dynfield->params));
+	mbuf_dynfield->offset = offset;
+	te->data = mbuf_dynfield;
+
+	TAILQ_INSERT_TAIL(mbuf_dynfield_list, te, next);
+
+	for (i = offset; i < offset + params->size; i++)
+		shm->free_space[i] = 0;
+	process_score();
+
+	RTE_LOG(DEBUG, MBUF, "Registered dynamic field %s (sz=%zu, al=%zu, fl=0x%x) -> %zd\n",
+		params->name, params->size, params->align, params->flags,
+		offset);
+
+	return offset;
+}
+
+int
+rte_mbuf_dynfield_register_offset(const struct rte_mbuf_dynfield *params,
+				size_t req)
+{
+	int ret;
+
+	if (params->size >= sizeof(struct rte_mbuf)) {
+		rte_errno = EINVAL;
+		return -1;
+	}
+	if (!rte_is_power_of_2(params->align)) {
+		rte_errno = EINVAL;
+		return -1;
+	}
+	if (params->flags != 0) {
+		rte_errno = EINVAL;
+		return -1;
+	}
+
+	rte_mcfg_tailq_write_lock();
+	ret = __rte_mbuf_dynfield_register_offset(params, req);
+	rte_mcfg_tailq_write_unlock();
+
+	return ret;
+}
+
+int
+rte_mbuf_dynfield_register(const struct rte_mbuf_dynfield *params)
+{
+	return rte_mbuf_dynfield_register_offset(params, SIZE_MAX);
+}
+
+/* assume tailq is locked */
+static struct mbuf_dynflag_elt *
+__mbuf_dynflag_lookup(const char *name)
+{
+	struct mbuf_dynflag_list *mbuf_dynflag_list;
+	struct mbuf_dynflag_elt *mbuf_dynflag;
+	struct rte_tailq_entry *te;
+
+	mbuf_dynflag_list = RTE_TAILQ_CAST(
+		mbuf_dynflag_tailq.head, mbuf_dynflag_list);
+
+	TAILQ_FOREACH(te, mbuf_dynflag_list, next) {
+		mbuf_dynflag = (struct mbuf_dynflag_elt *)te->data;
+		if (strncmp(name, mbuf_dynflag->params.name,
+				RTE_MBUF_DYN_NAMESIZE) == 0)
+			break;
+	}
+
+	if (te == NULL) {
+		rte_errno = ENOENT;
+		return NULL;
+	}
+
+	return mbuf_dynflag;
+}
+
+int
+rte_mbuf_dynflag_lookup(const char *name,
+			struct rte_mbuf_dynflag *params)
+{
+	struct mbuf_dynflag_elt *mbuf_dynflag;
+
+	if (shm == NULL) {
+		rte_errno = ENOENT;
+		return -1;
+	}
+
+	rte_mcfg_tailq_read_lock();
+	mbuf_dynflag = __mbuf_dynflag_lookup(name);
+	rte_mcfg_tailq_read_unlock();
+
+	if (mbuf_dynflag == NULL) {
+		rte_errno = ENOENT;
+		return -1;
+	}
+
+	if (params != NULL)
+		memcpy(params, &mbuf_dynflag->params, sizeof(*params));
+
+	return mbuf_dynflag->bitnum;
+}
+
+static int mbuf_dynflag_cmp(const struct rte_mbuf_dynflag *params1,
+		const struct rte_mbuf_dynflag *params2)
+{
+	if (strcmp(params1->name, params2->name))
+		return -1;
+	if (params1->flags != params2->flags)
+		return -1;
+	return 0;
+}
+
+/* assume tailq is locked */
+static int
+__rte_mbuf_dynflag_register_bitnum(const struct rte_mbuf_dynflag *params,
+				unsigned int req)
+{
+	struct mbuf_dynflag_list *mbuf_dynflag_list;
+	struct mbuf_dynflag_elt *mbuf_dynflag = NULL;
+	struct rte_tailq_entry *te = NULL;
+	unsigned int bitnum;
+	int ret;
+
+	if (shm == NULL && init_shared_mem() < 0)
+		return -1;
+
+	mbuf_dynflag = __mbuf_dynflag_lookup(params->name);
+	if (mbuf_dynflag != NULL) {
+		if (req != UINT_MAX && req != mbuf_dynflag->bitnum) {
+			rte_errno = EEXIST;
+			return -1;
+		}
+		if (mbuf_dynflag_cmp(params, &mbuf_dynflag->params) < 0) {
+			rte_errno = EEXIST;
+			return -1;
+		}
+		return mbuf_dynflag->bitnum;
+	}
+
+	if (rte_eal_process_type() != RTE_PROC_PRIMARY) {
+		rte_errno = EPERM;
+		return -1;
+	}
+
+	if (req == UINT_MAX) {
+		if (shm->free_flags == 0) {
+			rte_errno = ENOENT;
+			return -1;
+		}
+		bitnum = rte_bsf64(shm->free_flags);
+	} else {
+		if ((shm->free_flags & (1ULL << req)) == 0) {
+			rte_errno = EBUSY;
+			return -1;
+		}
+		bitnum = req;
+	}
+
+	mbuf_dynflag_list = RTE_TAILQ_CAST(
+		mbuf_dynflag_tailq.head, mbuf_dynflag_list);
+
+	te = rte_zmalloc("MBUF_DYNFLAG_TAILQ_ENTRY", sizeof(*te), 0);
+	if (te == NULL)
+		return -1;
+
+	mbuf_dynflag = rte_zmalloc("mbuf_dynflag", sizeof(*mbuf_dynflag), 0);
+	if (mbuf_dynflag == NULL) {
+		rte_free(te);
+		return -1;
+	}
+
+	ret = strlcpy(mbuf_dynflag->params.name, params->name,
+		sizeof(mbuf_dynflag->params.name));
+	if (ret < 0 || ret >= (int)sizeof(mbuf_dynflag->params.name)) {
+		rte_free(mbuf_dynflag);
+		rte_free(te);
+		rte_errno = ENAMETOOLONG;
+		return -1;
+	}
+	mbuf_dynflag->bitnum = bitnum;
+	te->data = mbuf_dynflag;
+
+	TAILQ_INSERT_TAIL(mbuf_dynflag_list, te, next);
+
+	shm->free_flags &= ~(1ULL << bitnum);
+
+	RTE_LOG(DEBUG, MBUF, "Registered dynamic flag %s (fl=0x%x) -> %u\n",
+		params->name, params->flags, bitnum);
+
+	return bitnum;
+}
+
+int
+rte_mbuf_dynflag_register_bitnum(const struct rte_mbuf_dynflag *params,
+				unsigned int req)
+{
+	int ret;
+
+	if (req >= RTE_SIZEOF_FIELD(struct rte_mbuf, ol_flags) * CHAR_BIT &&
+			req != UINT_MAX) {
+		rte_errno = EINVAL;
+		return -1;
+	}
+
+	rte_mcfg_tailq_write_lock();
+	ret = __rte_mbuf_dynflag_register_bitnum(params, req);
+	rte_mcfg_tailq_write_unlock();
+
+	return ret;
+}
+
+int
+rte_mbuf_dynflag_register(const struct rte_mbuf_dynflag *params)
+{
+	return rte_mbuf_dynflag_register_bitnum(params, UINT_MAX);
+}
+
+void rte_mbuf_dyn_dump(FILE *out)
+{
+	struct mbuf_dynfield_list *mbuf_dynfield_list;
+	struct mbuf_dynfield_elt *dynfield;
+	struct mbuf_dynflag_list *mbuf_dynflag_list;
+	struct mbuf_dynflag_elt *dynflag;
+	struct rte_tailq_entry *te;
+	size_t i;
+
+	rte_mcfg_tailq_write_lock();
+	init_shared_mem();
+	fprintf(out, "Reserved fields:\n");
+	mbuf_dynfield_list = RTE_TAILQ_CAST(
+		mbuf_dynfield_tailq.head, mbuf_dynfield_list);
+	TAILQ_FOREACH(te, mbuf_dynfield_list, next) {
+		dynfield = (struct mbuf_dynfield_elt *)te->data;
+		fprintf(out, "  name=%s offset=%zd size=%zd align=%zd flags=%x\n",
+			dynfield->params.name, dynfield->offset,
+			dynfield->params.size, dynfield->params.align,
+			dynfield->params.flags);
+	}
+	fprintf(out, "Reserved flags:\n");
+	mbuf_dynflag_list = RTE_TAILQ_CAST(
+		mbuf_dynflag_tailq.head, mbuf_dynflag_list);
+	TAILQ_FOREACH(te, mbuf_dynflag_list, next) {
+		dynflag = (struct mbuf_dynflag_elt *)te->data;
+		fprintf(out, "  name=%s bitnum=%u flags=%x\n",
+			dynflag->params.name, dynflag->bitnum,
+			dynflag->params.flags);
+	}
+	fprintf(out, "Free space in mbuf (0 = free, value = zone alignment):\n");
+	for (i = 0; i < sizeof(struct rte_mbuf); i++) {
+		if ((i % 8) == 0)
+			fprintf(out, "  %4.4zx: ", i);
+		fprintf(out, "%2.2x%s", shm->free_space[i],
+			(i % 8 != 7) ? " " : "\n");
+	}
+	rte_mcfg_tailq_write_unlock();
+}
diff --git a/lib/librte_mbuf/rte_mbuf_dyn.h b/lib/librte_mbuf/rte_mbuf_dyn.h
new file mode 100644
index 000000000..2e9d418cf
--- /dev/null
+++ b/lib/librte_mbuf/rte_mbuf_dyn.h
@@ -0,0 +1,239 @@
+/* SPDX-License-Identifier: BSD-3-Clause
+ * Copyright 2019 6WIND S.A.
+ */
+
+#ifndef _RTE_MBUF_DYN_H_
+#define _RTE_MBUF_DYN_H_
+
+/**
+ * @file
+ * RTE Mbuf dynamic fields and flags
+ *
+ * Many DPDK features require to store data inside the mbuf. As the room
+ * in mbuf structure is limited, it is not possible to have a field for
+ * each feature. Also, changing fields in the mbuf structure can break
+ * the API or ABI.
+ *
+ * This module addresses this issue, by enabling the dynamic
+ * registration of fields or flags:
+ *
+ * - a dynamic field is a named area in the rte_mbuf structure, with a
+ *   given size (>= 1 byte) and alignment constraint.
+ * - a dynamic flag is a named bit in the rte_mbuf structure, stored
+ *   in mbuf->ol_flags.
+ *
+ * The placement of the field or flag can be automatic, in this case the
+ * zones that have the smallest size and alignment constraint are
+ * selected in priority. Else, a specific field offset or flag bit
+ * number can be requested through the API.
+ *
+ * The typical use case is when a specific offload feature requires to
+ * register a dedicated offload field in the mbuf structure, and adding
+ * a static field or flag is not justified.
+ *
+ * Example of use:
+ *
+ * - A rte_mbuf_dynfield structure is defined, containing the parameters
+ *   of the dynamic field to be registered:
+ *   const struct rte_mbuf_dynfield rte_dynfield_my_feature = { ... };
+ * - The application initializes the PMD, and asks for this feature
+ *   at port initialization by passing DEV_RX_OFFLOAD_MY_FEATURE in
+ *   rxconf. This will make the PMD to register the field by calling
+ *   rte_mbuf_dynfield_register(&rte_dynfield_my_feature). The PMD
+ *   stores the returned offset.
+ * - The application that uses the offload feature also registers
+ *   the field to retrieve the same offset.
+ * - When the PMD receives a packet, it can set the field:
+ *   *RTE_MBUF_DYNFIELD(m, offset, <type *>) = value;
+ * - In the main loop, the application can retrieve the value with
+ *   the same macro.
+ *
+ * To avoid wasting space, the dynamic fields or flags must only be
+ * reserved on demand, when an application asks for the related feature.
+ *
+ * The registration can be done at any moment, but it is not possible
+ * to unregister fields or flags for now.
+ *
+ * A dynamic field can be reserved and used by an application only.
+ * It can for instance be a packet mark.
+ *
+ * To avoid namespace collisions, the dynamic mbuf field or flag names
+ * have to be chosen with care. It is advised to use the same
+ * conventions than function names in dpdk:
+ * - "rte_mbuf_dynfield_<name>" if defined in mbuf library
+ * - "rte_<libname>_dynfield_<name>" if defined in another library
+ * - "rte_net_<pmd>_dynfield_<name>" if defined in a in PMD
+ * - any name that does not start with "rte_" in an application
+ */
+
+#include <sys/types.h>
+/**
+ * Maximum length of the dynamic field or flag string.
+ */
+#define RTE_MBUF_DYN_NAMESIZE 64
+
+/**
+ * Structure describing the parameters of a mbuf dynamic field.
+ */
+struct rte_mbuf_dynfield {
+	char name[RTE_MBUF_DYN_NAMESIZE]; /**< Name of the field. */
+	size_t size;        /**< The number of bytes to reserve. */
+	size_t align;       /**< The alignment constraint (power of 2). */
+	unsigned int flags; /**< Reserved for future use, must be 0. */
+};
+
+/**
+ * Structure describing the parameters of a mbuf dynamic flag.
+ */
+struct rte_mbuf_dynflag {
+	char name[RTE_MBUF_DYN_NAMESIZE]; /**< Name of the dynamic flag. */
+	unsigned int flags; /**< Reserved for future use, must be 0. */
+};
+
+/**
+ * Register space for a dynamic field in the mbuf structure.
+ *
+ * If the field is already registered (same name and parameters), its
+ * offset is returned.
+ *
+ * @param params
+ *   A structure containing the requested parameters (name, size,
+ *   alignment constraint and flags).
+ * @return
+ *   The offset in the mbuf structure, or -1 on error.
+ *   Possible values for rte_errno:
+ *   - EINVAL: invalid parameters (size, align, or flags).
+ *   - EEXIST: this name is already register with different parameters.
+ *   - EPERM: called from a secondary process.
+ *   - ENOENT: not enough room in mbuf.
+ *   - ENOMEM: allocation failure.
+ *   - ENAMETOOLONG: name does not ends with \0.
+ */
+__rte_experimental
+int rte_mbuf_dynfield_register(const struct rte_mbuf_dynfield *params);
+
+/**
+ * Register space for a dynamic field in the mbuf structure at offset.
+ *
+ * If the field is already registered (same name, parameters and offset),
+ * the offset is returned.
+ *
+ * @param params
+ *   A structure containing the requested parameters (name, size,
+ *   alignment constraint and flags).
+ * @param offset
+ *   The requested offset. Ignored if SIZE_MAX is passed.
+ * @return
+ *   The offset in the mbuf structure, or -1 on error.
+ *   Possible values for rte_errno:
+ *   - EINVAL: invalid parameters (size, align, flags, or offset).
+ *   - EEXIST: this name is already register with different parameters.
+ *   - EBUSY: the requested offset cannot be used.
+ *   - EPERM: called from a secondary process.
+ *   - ENOENT: not enough room in mbuf.
+ *   - ENOMEM: allocation failure.
+ *   - ENAMETOOLONG: name does not ends with \0.
+ */
+__rte_experimental
+int rte_mbuf_dynfield_register_offset(const struct rte_mbuf_dynfield *params,
+				size_t offset);
+
+/**
+ * Lookup for a registered dynamic mbuf field.
+ *
+ * @param name
+ *   A string identifying the dynamic field.
+ * @param params
+ *   If not NULL, and if the lookup is successful, the structure is
+ *   filled with the parameters of the dynamic field.
+ * @return
+ *   The offset of this field in the mbuf structure, or -1 on error.
+ *   Possible values for rte_errno:
+ *   - ENOENT: no dynamic field matches this name.
+ */
+__rte_experimental
+int rte_mbuf_dynfield_lookup(const char *name,
+			struct rte_mbuf_dynfield *params);
+
+/**
+ * Register a dynamic flag in the mbuf structure.
+ *
+ * If the flag is already registered (same name and parameters), its
+ * bitnum is returned.
+ *
+ * @param params
+ *   A structure containing the requested parameters of the dynamic
+ *   flag (name and options).
+ * @return
+ *   The number of the reserved bit, or -1 on error.
+ *   Possible values for rte_errno:
+ *   - EINVAL: invalid parameters (size, align, or flags).
+ *   - EEXIST: this name is already register with different parameters.
+ *   - EPERM: called from a secondary process.
+ *   - ENOENT: no more flag available.
+ *   - ENOMEM: allocation failure.
+ *   - ENAMETOOLONG: name is longer than RTE_MBUF_DYN_NAMESIZE - 1.
+ */
+__rte_experimental
+int rte_mbuf_dynflag_register(const struct rte_mbuf_dynflag *params);
+
+/**
+ * Register a dynamic flag in the mbuf structure specifying bitnum.
+ *
+ * If the flag is already registered (same name, parameters and bitnum),
+ * the bitnum is returned.
+ *
+ * @param params
+ *   A structure containing the requested parameters of the dynamic
+ *   flag (name and options).
+ * @param bitnum
+ *   The requested bitnum. Ignored if UINT_MAX is passed.
+ * @return
+ *   The number of the reserved bit, or -1 on error.
+ *   Possible values for rte_errno:
+ *   - EINVAL: invalid parameters (size, align, or flags).
+ *   - EEXIST: this name is already register with different parameters.
+ *   - EBUSY: the requested bitnum cannot be used.
+ *   - EPERM: called from a secondary process.
+ *   - ENOENT: no more flag available.
+ *   - ENOMEM: allocation failure.
+ *   - ENAMETOOLONG: name is longer than RTE_MBUF_DYN_NAMESIZE - 1.
+ */
+__rte_experimental
+int rte_mbuf_dynflag_register_bitnum(const struct rte_mbuf_dynflag *params,
+				unsigned int bitnum);
+
+/**
+ * Lookup for a registered dynamic mbuf flag.
+ *
+ * @param name
+ *   A string identifying the dynamic flag.
+ * @param params
+ *   If not NULL, and if the lookup is successful, the structure is
+ *   filled with the parameters of the dynamic flag.
+ * @return
+ *   The offset of this flag in the mbuf structure, or -1 on error.
+ *   Possible values for rte_errno:
+ *   - ENOENT: no dynamic flag matches this name.
+ */
+__rte_experimental
+int rte_mbuf_dynflag_lookup(const char *name,
+			struct rte_mbuf_dynflag *params);
+
+/**
+ * Helper macro to access to a dynamic field.
+ */
+#define RTE_MBUF_DYNFIELD(m, offset, type) ((type)((uintptr_t)(m) + (offset)))
+
+/**
+ * Dump the status of dynamic fields and flags.
+ *
+ * @param out
+ *   The stream where the status is displayed.
+ */
+__rte_experimental
+void rte_mbuf_dyn_dump(FILE *out);
+
+/* Placeholder for dynamic fields and flags declarations. */
+
+#endif
diff --git a/lib/librte_mbuf/rte_mbuf_version.map b/lib/librte_mbuf/rte_mbuf_version.map
index a4f41d7fd..263dc0a21 100644
--- a/lib/librte_mbuf/rte_mbuf_version.map
+++ b/lib/librte_mbuf/rte_mbuf_version.map
@@ -58,6 +58,13 @@ EXPERIMENTAL {
 	global:
 
 	rte_mbuf_check;
+	rte_mbuf_dynfield_lookup;
+	rte_mbuf_dynfield_register;
+	rte_mbuf_dynfield_register_offset;
+	rte_mbuf_dynflag_lookup;
+	rte_mbuf_dynflag_register;
+	rte_mbuf_dynflag_register_bitnum;
+	rte_mbuf_dyn_dump;
 	rte_pktmbuf_copy;
 	rte_pktmbuf_free_bulk;
 
-- 
2.20.1