On 8/18/2025 4:11 PM, Morten Brørup wrote: > 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?
Maybe the pktmbuf which extbuf, which the data-room-size is zero when create pktmbuf. After detach from extbuf, the data_off should be zero instead of RTE_PKTMBUF_HEADROOM. > > 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 >
