RFC: Drop support for undersize packet mbufs

[Morten Brørup]

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 >= 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 = (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 = RTE_PKTMBUF_HEADROOM;

[3]: https://git.dpdk.org/dpdk/commit/lib/librte_mbuf?id=af75078fece3615088e561357c1e97603e43a5fe
+static inline void rte_pktmbuf_reset(struct rte_mbuf *m)
+{
+	uint32_t buf_ofs;
+
+	m->pkt.next = NULL;
+	m->pkt.pkt_len = 0;
+	m->pkt.l2_len = 0;
+	m->pkt.l3_len = 0;
+	m->pkt.vlan_tci = 0;
+	m->pkt.nb_segs = 1;
+	m->pkt.in_port = 0xff;
+
+	m->ol_flags = 0;
+	buf_ofs = (RTE_PKTMBUF_HEADROOM <= m->buf_len) ?
+			RTE_PKTMBUF_HEADROOM : m->buf_len;
+	m->pkt.data = (char*) m->buf_addr + buf_ofs;
+
+	m->pkt.data_len = 0;
+	__rte_mbuf_sanity_check(m, RTE_MBUF_PKT, 1);
+}



Med venlig hilsen / Kind regards,
-Morten Brørup