From mboxrd@z Thu Jan  1 00:00:00 1970
From: Maxime Coquelin <maxime.coquelin@redhat.com>
Subject: Re: [PATCH] vhost: Only access header if offloading is
 supported in dequeue path
Date: Thu, 6 Oct 2016 19:06:11 +0200
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On 10/06/2016 07:00 PM, Maxime Coquelin wrote:
> If offloading features are not negotiated, parsing the virtio header
> is not needed.
>
> Micro-benchmark with testpmd shows that the gain is +4% with indirect
> descriptors, +1% when using direct descriptors.
>
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  lib/librte_vhost/virtio_net.c | 47 ++++++++++++++++++++++++++++++++-----------
>  1 file changed, 35 insertions(+), 12 deletions(-)
>
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> index a59c39b..5d51693 100644
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -548,6 +548,18 @@ rte_vhost_enqueue_burst(int vid, uint16_t queue_id,
>  		return virtio_dev_rx(dev, queue_id, pkts, count);
>  }
>
> +static inline bool
> +virtio_net_with_host_offload(struct virtio_net *dev)
> +{
> +	if (dev->features &
> +			(VIRTIO_NET_F_CSUM | VIRTIO_NET_F_HOST_ECN |
> +			 VIRTIO_NET_F_HOST_TSO4 | VIRTIO_NET_F_HOST_TSO6 |
> +			 VIRTIO_NET_F_HOST_UFO))
> +		return true;
> +
> +	return false;
> +}
> +
>  static void
>  parse_ethernet(struct rte_mbuf *m, uint16_t *l4_proto, void **l4_hdr)
>  {
> @@ -600,6 +612,9 @@ vhost_dequeue_offload(struct virtio_net_hdr *hdr, struct rte_mbuf *m)
>  	void *l4_hdr = NULL;
>  	struct tcp_hdr *tcp_hdr = NULL;
>
> +	if (hdr->flags == 0 || hdr->gso_type == VIRTIO_NET_HDR_GSO_NONE)
> +		return;
> +

Oops, just noticed I forgot to amend a fix I did.
Of course, the above test should be:
if (hdr->flags == 0 && hdr->gso_type == VIRTIO_NET_HDR_GSO_NONE)

It will be fixed in the v2.

Regards,
Maxime