mbuf headroom question

[Olivier Matz]

On Thu, Jan 06, 2022 at 11:50:54AM +0100, Morten Brørup wrote:
> > From: Olivier Matz [mailto:olivier.matz at 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.

Well, not really ignore data_off. The application can use
rte_pktmbuf_prepend().

> > 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.

Yes. Currently, it is possible to use rte_pktmbuf_prepend(),
rte_pktmbuf_append(), and others on these mbufs. They will return
an error if data cannot be added.

> 
> It is probably not worth the effort pursuing this idea any further. :-)
> 
> > 
> > 
> > Olivier
>