Re: [PATCH] examples/vhost: fix perf regression

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Tue, Jul 19, 2016 at 01:53:11PM +0000, Jianfeng Tan wrote:
> 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: 859b480d5afd ("vhost: add guest offload setting")

And here you are patching vhost example to try to fix an "issue"
in vhost lib, this is __logically__ wrong.

	--yliu
> 
> Reported-by: Qian Xu <qian.q.xu@intel.com>
> Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
> ---
>  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