> From: Olivier Matz [mailto:olivier.m...@6wind.com] > Sent: Thursday, 6 January 2022 10.49
Thank you for the thorough explanation, Olivier. Somewhat exotic scenarios, but they do make sense! As you might have guessed, I was wondering if rte_pktmbuf_reset_headroom() could be optimized by simply using RTE_PKTMBUF_HEADROOM. I still think that it might, but I realize that it would have wider reaching consequences... > > Hi Morten, > > On Thu, Jan 06, 2022 at 10:29:11AM +0100, Morten Brørup wrote: > > Hi Olivier, > > > > The data_room_size parameter description for the mbuf pool creation > functions says: > > "Size of data buffer in each mbuf, including RTE_PKTMBUF_HEADROOM." > > > > Furthermore, both rte_mbuf_data_iova_default() and > rte_mbuf_data_addr_default() simply add RTE_PKTMBUF_HEADROOM to the > return value. > > > > Based on the above, I would think that it is impossible for m- > >buf_len to be smaller than RTE_PKTMBUF_HEADROOM. > > > > So why does rte_pktmbuf_reset_headroom() use RTE_MIN(m->buf_len, > RTE_PKTMBUF_HEADROOM), instead of just RTE_PKTMBUF_HEADROOM? What am I > missing here? > > It is legal to create a packet pool that has no data buffer: this pool > can be used to allocate packets clones that will be attached to mbufs > containing data. There is an example in test_mbuf.c. In this case, m->data_off is unused, and could be RTE_PKTMBUF_HEADROOM without causing problems. > > It is also technically possible to create a packet pool with small > mbufs (whose buffer length is less than RTE_PKTMBUF_HEADROOM). These > mbufs cannot be used by drivers which use rte_mbuf_data_iova_default(), > but they could be used internally. In this case, all of the mbuf's data buffer would be headroom, so the internal use be application/drivers would need to ignore m->data_ off anyway, and could be RTE_PKTMBUF_HEADROOM without causing problems. > > To create valid mbufs in these 2 cases, this is why RTE_MIN(m->buf_len, > RTE_PKTMBUF_HEADROOM) is used ; "valid" means that headroom is not > larger > than buffer length. Validity is important! So if we optimized rte_pktmbuf_reset_headroom(), all the related validation functions would need to be updated accordingly. And the description of the data_off field in the mbuf. It is probably not worth the effort pursuing this idea any further. :-) > > > Olivier
