Re: [dpdk-dev] [PATCH v4] net/i40e: relaxed barrier in the tx fastpath

-----Original Message-----
From: Gavin Hu <redacted>
Sent: Friday, February 14, 2020 3:58 PM
To: Gavin Hu <redacted>
Subject: Re: [dpdk-dev] [PATCH v4] net/i40e: relaxed barrier in the tx
fastpath

To keep ordering of mixed accesses, rte_cio is sufficient.
The rte_io barrier inside the I40E_PCI_REG_WRITE is overkill.[1]

[1] http://inbox.dpdk.org/dev/CALBAE1M-ezVWCjqCZDBw+MMDEC4O9
qf0Kpn89EMdGDajepKoZQ@mail.gmail.com

Fixes: 4861cde46116 ("i40e: new poll mode driver")
Cc: stable@dpdk.org

Signed-off-by: Gavin Hu <redacted>
---
V4:
- add the Fixes tag and CC stable <Xiaolong Ye>
V3:
- optimize the barriers in the fast path only, leave as it is for the
  barriers in the slow path and control path <jerin>
- drop the virtio patches from the list as they are in the control path
- it makes more sense to relax the barrier in the fast path, at the PMD level.
  relaxing the fundamental rte_io_x barriers APIs requires scrutinizations for
  each PMDs which use the barriers directly or indirectly.
V2:
- remove virtio_pci_read/write64 APIs definitions, they are not needed and
generate compiling errors like " error: unused function 'virtio_pci_write64' [-
Werror,-Wunused-function]"
- update the reference link to kernel source code
---
 drivers/net/i40e/i40e_rxtx.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/i40e/i40e_rxtx.c b/drivers/net/i40e/i40e_rxtx.c
index fd1ae80da..8c0f7cc67 100644
--- a/drivers/net/i40e/i40e_rxtx.c
+++ b/drivers/net/i40e/i40e_rxtx.c

@@ -1248,7 +1248,8 @@ i40e_xmit_pkts(void *tx_queue, struct rte_mbuf

**tx_pkts, uint16_t nb_pkts)
    (unsigned) txq->port_id, (unsigned) txq->queue_id,
    (unsigned) tx_id, (unsigned) nb_tx);

-I40E_PCI_REG_WRITE(txq->qtx_tail, tx_id);
+rte_cio_wmb();
+I40E_PCI_REG_WRITE_RELAXED(txq->qtx_tail, tx_id);
 txq->tx_tail = tx_id;

 return nb_tx;
--
2.17.1