Re: RFC: Drop support for undersize packet mbufs

[fengchengwen <fengchengwen@huawei.com>]

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
>