Re: [dpdk-dev] [PATCH 2/2] Remove RTE_MBUF_REFCNT references

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi lads,

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
> Sent: Wednesday, February 18, 2015 9:36 AM
> To: Olivier MATZ
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH 2/2] Remove RTE_MBUF_REFCNT references
> 
> On Wed, Feb 18, 2015 at 10:16:56AM +0100, Olivier MATZ wrote:
> > 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@intel.com>
> >
> > 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.
> >
> 
> While I understand how the macros make certain assumptions, how does activating
> the refcnt specifically lead to the problems you describe? Could you explain
> that part in a bit more detail?
> 
> Thanks,
> /Bruce
> 

Olivier, I also don't understand your concern here.
As I can see, that patch has nothing to do with RTE_MBUF_FROM_BADDR/ RTE_MBUF_FROM_BADDR macros.
They are still there, for example rte_pktmbuf_detach() still uses it to restore mbuf's buf_addr.
The only principal change here, is that we don't rely more  on RTE_MBUF_FROM_BADDR to determine,
Is that indirect mbuf or not. 
Instead we use a special falg for that purpose:

-#define RTE_MBUF_INDIRECT(mb)   (RTE_MBUF_FROM_BADDR((mb)->buf_addr) != (mb))
+#define RTE_MBUF_INDIRECT(mb)   (mb->ol_flags & IND_ATTACHED_MBUF)
 
BTW, Sergio as I said before, I think it should be:
#define RTE_MBUF_INDIRECT(mb)   ((mb)->ol_flags & IND_ATTACHED_MBUF)

Konstantin


> > 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
> >