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

[Gavin Hu <Gavin.Hu@arm.com>]

Hi Jerin,

Could you help review this patch?  We discussed a lot on this topic and I think we agreed already to relax the barrier only on the fast path.

Best Regards,
Gavin


> -----Original Message-----
> From: Gavin Hu <Gavin.Hu@arm.com>
> Sent: Friday, February 14, 2020 3:58 PM
> To: Gavin Hu <Gavin.Hu@arm.com>
> 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 <gavin.hu@arm.com>
> ---
> 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
>