diff options
authorJianfeng Tan <>2016-07-21 00:42:45 +0000
committerThomas Monjalon <>2016-07-22 12:40:26 +0200
commit56c9ff8e3e53bb69c3b0b59454b313e075e2fab4 (patch)
parentb86eeb2aba2bd7c0fc22a317bcf7cfb5e3541e06 (diff)
examples/vhost: fix performance
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 commit, 859b480d5afd ("vhost: add guest offload setting"), adds support for VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6, in vhost lib. But inside vhost example, the way to disable tso only excludes the direction from virtio to vhost, but not the opposite direction. When mergeable is disabled, it triggers big_packets path of virtio-net driver to prepare to receive possible big packets with size of 64K. Because mergeable is off, for each entry of avail ring, virtio driver uses 19 desc chained together, with one desc pointing to header, other 18 desc pointing to 4K-sized pages. 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 <> Signed-off-by: Jianfeng Tan <> Tested-by: Qian Xu <> Acked-by: Yuanhan Liu <>
1 files changed, 2 insertions, 0 deletions
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;