Re: [PATCH 2/2] Remove RTE_MBUF_REFCNT references

[Olivier MATZ <olivier.matz-pdR9zngts4EAvxtiuMwx3w@public.gmane.org>]

Hi Sergio,

On 02/16/2015 05:08 PM, Sergio Gonzalez Monroy wrote:
> This patch removes all references to RTE_MBUF_REFCNT, setting the refcnt
> field in the mbuf struct permanently.
>
> Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>

I think removing the refcount compile option goes in the right
direction. However, activating the refcount will break the applications
that reserve a private zone in mbufs. This is due to the macros
RTE_MBUF_TO_BADDR() and RTE_MBUF_FROM_BADDR() that suppose that
the beginning of the mbuf is 128 bytes (sizeof mbuf) before the
data buffer.

For RTE_MBUF_TO_BADDR(), it's relatively easy to replace it. The
mbuf pool could store the size of the private size like it's done
for mbp_priv->mbuf_data_room_size. Using rte_mempool_from_obj(m)
or m->pool, we can retrieve the mbuf pool and this value, then
compute the buffer address.

For RTE_MBUF_FROM_BADDR(), it's more complex. We could ensure that
a backpointer to the mbuf is always located before the data buffer,
but it looks difficult to do.

Another idea would be to add a field in indirect mbufs that stores
the pointer to the "parent" mbuf.

Regards,
Olivier