macvtap performance regression (bisected) between 3.13 and 3.14-rc1

[Christian Borntraeger]

Vlad,

commit 6acf54f1cf0a6747bac9fea26f34cfc5a9029523
    macvtap: Add support of packet capture on macvtap device.

causes a performance regression for iperf traffic between two KVM guests
on my s390 system. Both guests are connected via two macvtaps on the same OSA
network card.
Before that patch I get ~20 Gbit/sec between two guests, afterwards I get
~4Gbit/sec

Latency seems to be unchanges (uperf 1byte ping pong).

According to ifconfig in the guest, I have ~ 1500 bytes per packet with this
patch and ~  40000 bytes without. So for some reason this patch causes the
network stack to do segmentation. (the guest kernel stays the same, only host 
kernel is changed).

Any ideas?


Christian

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