Re: [dpdk-dev] [PATCH v3] net/i40e: relaxed barrier in the tx fastpath

[Jerin Jacob <jerinjacobk@gmail.com>]

On Sat, Feb 8, 2020 at 7:22 PM Gavin Hu <gavin.hu@arm.com> wrote:
>
> To keep ordering of mixed accesses, rte_cio is sufficient.
> The rte_io barrier is overkill.[1]
>
> [1] http://inbox.dpdk.org/dev/CALBAE1M-ezVWCjqCZDBw+MMDEC4O9
> qf0Kpn89EMdGDajepKoZQ@mail.gmail.com
>
> Signed-off-by: Gavin Hu <gavin.hu@arm.com>
> ---
> 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);

Looks good to me from the ARM64 perspective or in general.

Reviewed-by: Jerin Jacob <jerinj@marvell.com>





>         txq->tx_tail = tx_id;
>
>         return nb_tx;
> --
> 2.17.1
>