Re: [dpdk-dev] [PATCH v2] raw/ntb: fix write memory barrier issue

["Li, Xiaoyun" <xiaoyun.li@intel.com>]

Hi
I reconsidered and retested about this issue.
I still need to use rte_wmb instead of using rte_io_wmb.

Because to achieve high performance, ntb needs to turn on WC(write combining) feature. The perf difference with and without WC enabled is more than 20X.
And when WC enabled, rte_io_wmb cannot make sure the instructions are in order only rte_wmb can make sure that.

And in my retest, when sending 64 bytes packets, using rte_io_wmb will cause out-of-order issue and cause memory corruption on rx side.
And using rte_wmb is fine.

So I can only use v1 patch and suspend v2 patch in patchwork.

Best Regards
Xiaoyun Li

> -----Original Message-----
> From: Gavin Hu (Arm Technology China) [mailto:Gavin.Hu@arm.com]
> Sent: Monday, December 16, 2019 18:50
> To: Li, Xiaoyun <xiaoyun.li@intel.com>; Wu, Jingjing <jingjing.wu@intel.com>
> Cc: dev@dpdk.org; Maslekar, Omkar <omkar.maslekar@intel.com>;
> stable@dpdk.org; nd <nd@arm.com>
> Subject: RE: [dpdk-dev] [PATCH v2] raw/ntb: fix write memory barrier issue
> 
> 
> 
> > -----Original Message-----
> > From: dev <dev-bounces@dpdk.org> On Behalf Of Xiaoyun Li
> > Sent: Monday, December 16, 2019 9:59 AM
> > To: jingjing.wu@intel.com
> > Cc: dev@dpdk.org; omkar.maslekar@intel.com; Xiaoyun Li
> > <xiaoyun.li@intel.com>; stable@dpdk.org
> > Subject: [dpdk-dev] [PATCH v2] raw/ntb: fix write memory barrier issue
> >
> > All buffers and ring info should be written before tail register update.
> > This patch relocates the write memory barrier before updating tail
> > register to avoid potential issues.
> >
> > Fixes: 11b5c7daf019 ("raw/ntb: add enqueue and dequeue functions")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Xiaoyun Li <xiaoyun.li@intel.com>
> > ---
> > v2:
> >  * Replaced rte_wmb with rte_io_wmb since rte_io_wmb is enough.
> > ---
> >  drivers/raw/ntb/ntb.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/raw/ntb/ntb.c b/drivers/raw/ntb/ntb.c index
> > ad7f6abfd..c7de86f36 100644
> > --- a/drivers/raw/ntb/ntb.c
> > +++ b/drivers/raw/ntb/ntb.c
> > @@ -683,8 +683,8 @@ ntb_enqueue_bufs(struct rte_rawdev *dev,
> >  			   sizeof(struct ntb_used) * nb1);
> >  		rte_memcpy(txq->tx_used_ring, tx_used + nb1,
> >  			   sizeof(struct ntb_used) * nb2);
> > +		rte_io_wmb();
> As both txq->tx_used_ring and *txq->used_cnt are physically reside in the PCI
> device side, rte_io_wmb is correct to ensure the ordering.
> 
> >  		*txq->used_cnt = txq->last_used;
> > -		rte_wmb();
> >
> >  		/* update queue stats */
> >  		hw->ntb_xstats[NTB_TX_BYTES_ID + off] += bytes; @@ -789,8
> +789,8 @@
> > ntb_dequeue_bufs(struct rte_rawdev *dev,
> >  			   sizeof(struct ntb_desc) * nb1);
> >  		rte_memcpy(rxq->rx_desc_ring, rx_desc + nb1,
> >  			   sizeof(struct ntb_desc) * nb2);
> > +		rte_io_wmb();
> >  		*rxq->avail_cnt = rxq->last_avail;
> > -		rte_wmb();
> >
> >  		/* update queue stats */
> >  		off = NTB_XSTATS_NUM * ((size_t)context + 1);
> > --
> > 2.17.1
> 
> Reviewed-by: Gavin Hu <gavin.hu@arm.com>