Re: [dpdk-dev] [PATCH 1/2] ixgbe: fix setup of mbuf initializer template

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

> -----Original Message-----
> From: Jean-Mickael Guerin [mailto:jean-mickael.guerin@6wind.com]
> Sent: Thursday, December 04, 2014 3:15 PM
> To: Ananyev, Konstantin; Richardson, Bruce
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH 1/2] ixgbe: fix setup of mbuf initializer template
> 
> On 04/12/2014 15:42, Ananyev, Konstantin wrote:
> >
> >
> >> -----Original Message-----
> >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
> >> Sent: Thursday, December 04, 2014 2:40 PM
> >> To: Jean-Mickael Guerin
> >> Cc: dev@dpdk.org
> >> Subject: Re: [dpdk-dev] [PATCH 1/2] ixgbe: fix setup of mbuf initializer template
> >>
> >> On Thu, Dec 04, 2014 at 03:26:20PM +0100, Jean-Mickael Guerin wrote:
> >>> Add a compiler barrier to make sure all fields covered by
> >>> the marker rearm_data are assigned before the read.
> >>>
> >>> Signed-off-by: Jean-Mickael Guerin <jean-mickael.guerin@6wind.com>
> >>> Acked-by: David Marchand <david.marchand@6wind.com>
> >>> Fixes: 0ff3324da2 ("ixgbe: rework vector pmd following mbuf changes")
> >>
> >> Acked-by: Bruce Richardson <bruce.richardson@intel.com>
> >>
> >>> ---
> >>>   lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c | 3 +++
> >>>   1 file changed, 3 insertions(+)
> >>>
> >>> diff --git a/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c b/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
> >>> index 579bc46..c1b5a78 100644
> >>> --- a/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
> >>> +++ b/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
> >>> @@ -739,6 +739,9 @@ ixgbe_rxq_vec_setup(struct igb_rx_queue *rxq)
> >>>   	mb_def.buf_len = rxq->mb_pool->elt_size - sizeof(struct rte_mbuf);
> >>>   	mb_def.port = rxq->port_id;
> >>>   	rte_mbuf_refcnt_set(&mb_def, 1);
> >>> +
> >>> +	/* prevent compiler reordering: rearm_data covers previous fields */
> >>> +	rte_compiler_barrier();
> >>>   	rxq->mbuf_initializer = *((uint64_t *)&mb_def.rearm_data);
> >>>   	return 0;
> >>>   }
> >>> --
> >
> > Hmm, can someone explain to me why do we need a compiler barrier here?
> > Konstantin
> 
> rearm_data is a separate field and as well an array of length zero,
> overlapping on purpose the fields data_off buf_len, port, refcnt.
> It might depend on compiler, but I could see a wrong value of 0UL for
> mbuf_initializer without the barrier (gcc 4.4.6).

Ah ok then.
Probably it is some sort of bug in old version of the gcc.

> 
> 
> >
> >>> 2.1.3
> >>>