[dpdk-dev,v2] examples/vhost: fix perf regression

  We find significant perfermance drop introduced by below commit,
when vhost example is started with --mergeable 0 and inside vm,
kernel virtio-net driver is used to do ip based forwarding.

The root cause is that below commit adds support for
VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6, and when
mergeable is disabled, it triggers big_packets path of virtio-net
driver. In this path, virtio driver uses 19 desc with 18 4K-sized
pages to receive each packet, so that it can receive a big packet
with size of 64K. But QEMU only creates 256 desc entries for each
vq, which results in that only 13 packets can be received. VM
kernel can quickly handle those packets and go to sleep (HLT).

As QEMU has no option to set the desc entries of a vq, so here,
we disable VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6
with VIRTIO_NET_F_HOST_TSO4 and VIRTIO_NET_F_HOST_TSO6 when we
disable tso of vhost example, to avoid VM kernel virtio driver
go into big_packets path.

Fixes: 9fd72e3cbd29 ("examples/vhost: add virtio offload")

Reported-by: Qian Xu <qian.q.xu@intel.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
---
v2: change the Fixes line to point to proper commit to fix.
 examples/vhost/main.c | 2 ++
 1 file changed, 2 insertions(+)
  

Self-Nack this patch because the commit log needs change further.

> -----Original Message-----
> From: Tan, Jianfeng
> Sent: Thursday, July 21, 2016 8:24 AM
> To: dev@dpdk.org
> Cc: yuanhan.liu@linux.intel.com; Wang, Zhihong; Xu, Qian Q; Tan, Jianfeng
> Subject: [PATCH v2] examples/vhost: fix perf regression
> 
> We find significant perfermance drop introduced by below commit,
> when vhost example is started with --mergeable 0 and inside vm,
> kernel virtio-net driver is used to do ip based forwarding.
> 
> The root cause is that below commit adds support for
> VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6, and when
> mergeable is disabled, it triggers big_packets path of virtio-net
> driver. In this path, virtio driver uses 19 desc with 18 4K-sized
> pages to receive each packet, so that it can receive a big packet
> with size of 64K. But QEMU only creates 256 desc entries for each
> vq, which results in that only 13 packets can be received. VM
> kernel can quickly handle those packets and go to sleep (HLT).
> 
> As QEMU has no option to set the desc entries of a vq, so here,
> we disable VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6
> with VIRTIO_NET_F_HOST_TSO4 and VIRTIO_NET_F_HOST_TSO6 when we
> disable tso of vhost example, to avoid VM kernel virtio driver
> go into big_packets path.
> 
> Fixes: 9fd72e3cbd29 ("examples/vhost: add virtio offload")
> 
> Reported-by: Qian Xu <qian.q.xu@intel.com>
> Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
> ---
> v2: change the Fixes line to point to proper commit to fix.
>  examples/vhost/main.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/examples/vhost/main.c b/examples/vhost/main.c
> index 3b98f42..92a9823 100644
> --- a/examples/vhost/main.c
> +++ b/examples/vhost/main.c
> @@ -327,6 +327,8 @@ port_init(uint8_t port)
>  	if (enable_tso == 0) {
>  		rte_vhost_feature_disable(1ULL <<
> VIRTIO_NET_F_HOST_TSO4);
>  		rte_vhost_feature_disable(1ULL <<
> VIRTIO_NET_F_HOST_TSO6);
> +		rte_vhost_feature_disable(1ULL <<
> VIRTIO_NET_F_GUEST_TSO4);
> +		rte_vhost_feature_disable(1ULL <<
> VIRTIO_NET_F_GUEST_TSO6);
>  	}
> 
>  	rx_rings = (uint16_t)dev_info.max_rx_queues;
> --
> 2.7.4