RE: mbuf headroom question

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Olivier Matz [mailto:olivier.matz@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