port-xen/50290: Regression: xennet very slow in netbsd-7

[netbsd%precedence.co.uk@localhost]

>Number:         50290
>Category:       port-xen
>Synopsis:       Regression: xennet very slow in netbsd-7
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-xen-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Sep 29 09:55:00 +0000 2015
>Originator:     Stephen Borrill
>Release:        7.0_RC3
>Organization:
>Environment:
NetBSD nmdevel7 7.0_RC3 NetBSD 7.0_RC3 (NETMANXEN3) #24: Mon Sep  7 11:38:06 BST 2015  root@builder7:/usr/work/netmanager/work/obj/7.0/sys/arch/amd64/compile/NETMANXEN3 amd64
>Description:
Network throughput with the xennet driver on netbsd-7 is 10% of netbsd-5. Testbed is physical netbsd-5 host as netio client, with netio servers runs on netbsd-5 and netbsd-7 Xen domUs on the same XenServer host.

NetBSD 5.2 almost saturates the 1Gb link:
TCP connection established.
Packet size  1k bytes:  103375 KByte/s Tx,  81319 KByte/s Rx.
Packet size  2k bytes:  107507 KByte/s Tx,  96727 KByte/s Rx.
Packet size  4k bytes:  110407 KByte/s Tx,  94854 KByte/s Rx.
Packet size  8k bytes:  111326 KByte/s Tx,  108494 KByte/s Rx.
Packet size 16k bytes:  111244 KByte/s Tx,  107681 KByte/s Rx.
Packet size 32k bytes:  111291 KByte/s Tx,  99243 KByte/s Rx.

NetBSD 7.0_RC3 has 10% of throughput:
Packet size  1k bytes:  9641 KByte/s Tx,  9890 KByte/s Rx.
Packet size  2k bytes:  11940 KByte/s Tx,  11718 KByte/s Rx.
Packet size  4k bytes:  11529 KByte/s Tx,  12110 KByte/s Rx.
Packet size  8k bytes:  11485 KByte/s Tx,  12266 KByte/s Rx.
Packet size 16k bytes:  11435 KByte/s Tx,  12051 KByte/s Rx.
Packet size 32k bytes:  11501 KByte/s Tx,  12287 KByte/s Rx.

>How-To-Repeat:
As above
>Fix: