Re: [dpdk-dev] [PATCH 1/2] net/i40e: fix risk in Rx descriptor read in NEON vector path

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>
Similar comments that I have to patch 2/2

> 
> Rx descriptor is 16B/32B in size and consists of multiple words.
> The word that includes DD field should be read first. Read result with DD bit
> set indicates the rest part in a descriptor is valid.
Suggest rewording as follows:
Rx descriptor is 16B/32B in size. If the DD bit is set, it indicates that the rest of the descriptor words have valid values. Hence, the word containing DD bit must be read first before reading the rest of the descriptor words.

> 
> In NEON vector PMD, vector load loads two contiguous 8B of descriptor data
> into vector register. Given vector load ensures no 16B atomicity, read of the
> word that includes DD field could be reordered after read of other words. In
> this case, some words could be invalid data.
"some words could contain invalid data"

> 
> Read barrier is added after read of qword1 that includes DD field.
> And qword0 is reloaded to update vector register. This ensures what fetched
> is correct descriptor data.
"This ensures that the fetched data is correct".

Suggest capturing the performance impact, so it is clearly documented.
> 
> Fixes: ae0eb310f253 ("net/i40e: implement vector PMD for ARM")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
With the above comments,
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

> ---
>  drivers/net/i40e/i40e_rxtx_vec_neon.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/net/i40e/i40e_rxtx_vec_neon.c
> b/drivers/net/i40e/i40e_rxtx_vec_neon.c
> index b2683fda60..71191c7cc8 100644
> --- a/drivers/net/i40e/i40e_rxtx_vec_neon.c
> +++ b/drivers/net/i40e/i40e_rxtx_vec_neon.c
> @@ -286,6 +286,14 @@ _recv_raw_pkts_vec(struct i40e_rx_queue
> *__rte_restrict rxq,
>  		descs[1] =  vld1q_u64((uint64_t *)(rxdp + 1));
>  		descs[0] =  vld1q_u64((uint64_t *)(rxdp));
> 
> +		/* Use acquire fence to order loads of descriptor qwords */
> +		rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
> +		/* A.2 reload qword0 to make it ordered after qword1 load */
> +		descs[3] = vld1q_lane_u64((uint64_t *)(rxdp + 3), descs[3],
> 0);
> +		descs[2] = vld1q_lane_u64((uint64_t *)(rxdp + 2), descs[2],
> 0);
> +		descs[1] = vld1q_lane_u64((uint64_t *)(rxdp + 1), descs[1],
> 0);
> +		descs[0] = vld1q_lane_u64((uint64_t *)(rxdp), descs[0], 0);
> +
>  		/* B.1 load 4 mbuf point */
>  		mbp1 = vld1q_u64((uint64_t *)&sw_ring[pos]);
>  		mbp2 = vld1q_u64((uint64_t *)&sw_ring[pos + 2]);
> --
> 2.25.1