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. 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. 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. Olivier
