[PATCH 19.11] net/i40e: fix risk in descriptor read in NEON Rx

[Ruifeng Wang <ruifeng.wang@arm.com>]

[ upstream commit 778602fe570a138224de94a38eca3ce2e344138c ]

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
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
that the fetched data is correct.

Testpmd single core test on N1SDP/ThunderX2 showed no performance drop.

Fixes: ae0eb310f253 ("net/i40e: implement vector PMD for ARM")

Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
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 bd1e0490d..0da6b37da 100644
--- a/drivers/net/i40e/i40e_rxtx_vec_neon.c
+++ b/drivers/net/i40e/i40e_rxtx_vec_neon.c
@@ -299,6 +299,14 @@ _recv_raw_pkts_vec(struct i40e_rx_queue *rxq, struct rte_mbuf **rx_pkts,
 		/* B.2 copy 2 mbuf point into rx_pkts  */
 		vst1q_u64((uint64_t *)&rx_pkts[pos + 2], mbp2);
 
+		/* Use acquire fence to order loads of descriptor qwords */
+		__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);
+
 		if (split_packet) {
 			rte_mbuf_prefetch_part2(rx_pkts[pos]);
 			rte_mbuf_prefetch_part2(rx_pkts[pos + 1]);
-- 
2.25.1