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

[Gavin Hu (Arm Technology China)]

Hi Xiaoyun,

> -----Original Message-----
> From: dev <dev-boun...@dpdk.org> On Behalf Of Xiaoyun Li
> Sent: Wednesday, December 4, 2019 11:19 PM
> To: jingjing...@intel.com
> Cc: dev@dpdk.org; Xiaoyun Li <xiaoyun...@intel.com>; sta...@dpdk.org
> Subject: [dpdk-dev] [PATCH] 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: sta...@dpdk.org
> 
> Signed-off-by: Xiaoyun Li <xiaoyun...@intel.com>
> ---
>  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..dd0b72f8c 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);
> -             *txq->used_cnt = txq->last_used;
>               rte_wmb();
> +             *txq->used_cnt = txq->last_used;
I am ok with the re-location of the barrier, but why not the rte_io_wmb instead 
of rte_wmb? 
Rte_io_wmb is sufficient to guarantee the preceding stores are visible to the 
device, rte_wmb is overkill. 
https://code.dpdk.org/dpdk/latest/source/lib/librte_eal/common/include/generic/rte_atomic.h#L92
 
> 
>               /* 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);
> -             *rxq->avail_cnt = rxq->last_avail;
>               rte_wmb();
> +             *rxq->avail_cnt = rxq->last_avail;
> 
>               /* update queue stats */
>               off = NTB_XSTATS_NUM * ((size_t)context + 1);
> --
> 2.17.1