Re: [dpdk-dev] [PATCH] virtio: tx with can_push when VERSION_1 is set

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Pierre,

On 11/08/2016 10:31 AM, Pierre Pfister (ppfister) wrote:
> Current virtio driver advertises VERSION_1 support,
> but does not handle device's VERSION_1 support when
> sending packets (it looks for ANY_LAYOUT feature,
> which is absent).
>
> This patch enables 'can_push' in tx path when VERSION_1
> is advertised by the device.
>
> This significantly improves small packets forwarding rate
> towards devices advertising VERSION_1 feature.
I think it depends whether offloading is enabled or not.
If no offloading enabled, I measured significant drop.
Indeed, when no offloading is enabled, the Tx path in Virtio
does not access the virtio header before your patch, as the header is 
memset to zero at device init time.
With your patch, it gets memset to zero at every transmit in the hot
path.

With offloading enabled, it does makes sense though, as the header will
be accessed.

This patch is for v17.02 anyway, and we may provide a way to enable and
disable features at Virtio PMD init time by this release.

Thanks,
Maxime

>
> Signed-off-by: Pierre Pfister <ppfister@cisco.com>
> ---
>  drivers/net/virtio/virtio_rxtx.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> index 724517e..2fe0338 100644
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -925,7 +925,8 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
>                 }
>
>                 /* optimize ring usage */
> -               if (vtpci_with_feature(hw, VIRTIO_F_ANY_LAYOUT) &&
> +               if ((vtpci_with_feature(hw, VIRTIO_F_ANY_LAYOUT) ||
> +                     vtpci_with_feature(hw, VIRTIO_F_VERSION_1)) &&
>                     rte_mbuf_refcnt_read(txm) == 1 &&
>                     RTE_MBUF_DIRECT(txm) &&
>                     txm->nb_segs == 1 &&
> --
> 2.7.4 (Apple Git-66)
>