Performance regression 3.11 with macvlan between 2 linux guests (bisected)

Vlad,

the patch 

commit 3e4f8b787370978733ca6cae452720a4f0c296b8
Author: Vlad Yasevich [off-list ref]
Date:   Tue Jun 25 16:04:22 2013 -0400

    macvtap: Perform GSO on forwarding path.

causes a severe performance regression for 2 Linux guests with virtio-net
connected via macvlan/vtap doing iperf workload. (2GBit vs. 20Gbit)

If I understand the patch correctly, we now check for gso depending on the
macvlan features. If the underlying hardware does not support the necessary
offloads then we will do segmentation, even if we keep the whole traffic internal
between two guests and even if both guests supports LRO,GSO etc.
---snip---
[...]
+       features = netif_skb_features(skb) & vlan->tap_features;
+       if (netif_needs_gso(skb, features)) {
+               struct sk_buff *segs = __skb_gso_segment(skb, features, false);
[...]

---snip---
Shouldnt we take the features of the target device or even better do the gsoing in the
target device driver?

Christian

FYI, the underlying HW has:

Features for eth0:
rx-checksumming: off [fixed]
tx-checksumming: off
	tx-checksum-ipv4: off [fixed]
	tx-checksum-ip-generic: off [fixed]
	tx-checksum-ipv6: off [fixed]
	tx-checksum-fcoe-crc: off [fixed]
	tx-checksum-sctp: off [fixed]
scatter-gather: off
	tx-scatter-gather: off [fixed]
	tx-scatter-gather-fraglist: off [fixed]
tcp-segmentation-offload: off
	tx-tcp-segmentation: off [fixed]
	tx-tcp-ecn-segmentation: off [fixed]
	tx-tcp6-segmentation: off [fixed]
udp-fragmentation-offload: off [fixed]
generic-segmentation-offload: off [requested on]
generic-receive-offload: on
large-receive-offload: off [fixed]
rx-vlan-offload: off [fixed]
tx-vlan-offload: off [fixed]
ntuple-filters: off [fixed]
receive-hashing: off [fixed]
highdma: off [fixed]
rx-vlan-filter: on [fixed]
vlan-challenged: off [fixed]
tx-lockless: off [fixed]
netns-local: off [fixed]
tx-gso-robust: off [fixed]
tx-fcoe-segmentation: off [fixed]
tx-gre-segmentation: off [fixed]
tx-udp_tnl-segmentation: off [fixed]
tx-mpls-segmentation: off [fixed]
fcoe-mtu: off [fixed]
tx-nocache-copy: off
loopback: off [fixed]
rx-fcs: off [fixed]
rx-all: off [fixed]
tx-vlan-stag-hw-insert: off [fixed]
rx-vlan-stag-hw-parse: off [fixed]
rx-vlan-stag-filter: off [fixed]