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Subject: RFC: Drop support for undersize packet mbufs
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Date: Mon, 18 Aug 2025 10:11:46 +0200
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Thread-Topic: RFC: Drop support for undersize packet mbufs
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From: =?iso-8859-1?Q?Morten_Br=F8rup?= <mb@smartsharesystems.com>
To: <thomas@monjalon.net>, <bruce.richardson@intel.com>,
 <stephen@networkplumber.org>, <andrew.rybchenko@oktetlabs.ru>
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Why does the mbuf library support packet mbufs with smaller data room =
size than RTE_PKTMBUF_HEADROOM (e.g. [1]), when the Ethdev drivers (e.g. =
[2]) don't support it?

This goes all the way back to the first public release [3].

It seems crazy exotic, and should be removed for simplicity and a =
potential performance benefit.
What am I missing here?

Instead, the rte_pktmbuf_pool_create() functions should check that =
data_room_size >=3D RTE_PKTMBUF_HEADROOM.


[1]: =
https://elixir.bootlin.com/dpdk/v25.07/source/lib/mbuf/rte_mbuf.h#L941
static inline void rte_pktmbuf_reset_headroom(struct rte_mbuf *m)
{
	m->data_off =3D (uint16_t)RTE_MIN((uint16_t)RTE_PKTMBUF_HEADROOM,
					(uint16_t)m->buf_len);
}

[2]: =
https://elixir.bootlin.com/dpdk/v25.07/source/drivers/net/intel/i40e/i40e=
_rxtx.c#L609
mb->data_off =3D RTE_PKTMBUF_HEADROOM;

[3]: =
https://git.dpdk.org/dpdk/commit/lib/librte_mbuf?id=3Daf75078fece3615088e=
561357c1e97603e43a5fe
+static inline void rte_pktmbuf_reset(struct rte_mbuf *m)
+{
+	uint32_t buf_ofs;
+
+	m->pkt.next =3D NULL;
+	m->pkt.pkt_len =3D 0;
+	m->pkt.l2_len =3D 0;
+	m->pkt.l3_len =3D 0;
+	m->pkt.vlan_tci =3D 0;
+	m->pkt.nb_segs =3D 1;
+	m->pkt.in_port =3D 0xff;
+
+	m->ol_flags =3D 0;
+	buf_ofs =3D (RTE_PKTMBUF_HEADROOM <=3D m->buf_len) ?
+			RTE_PKTMBUF_HEADROOM : m->buf_len;
+	m->pkt.data =3D (char*) m->buf_addr + buf_ofs;
+
+	m->pkt.data_len =3D 0;
+	__rte_mbuf_sanity_check(m, RTE_MBUF_PKT, 1);
+}



Med venlig hilsen / Kind regards,
-Morten Br=F8rup