virtio net regression

[Antoine Martin <antoine@devloop.org.uk>]

Hi,

I've got some hosts that were happily running the 2.6.25.x host kernel,
kvm-84, kernel.org kvm modules.
The guests were running 2.6.25 to 2.6.29.x quite happily.
Network was using virtio.
Since I upgraded one of the hosts (Intel dual core) to 2.6.29.x
yesterday, the virtio network performance of the guests on it dropped
dramatically. (for some reason another AMD host did not seem to be
affected...)
Here are the tests I performed using wget and scp:
* guest to guest: fast
* guest to host: fast
* host to internet: fast
* guest to internet: slow!!!
I was normally getting ~5MB/s to the host (speed to the internet was
limited by the capacity of the DSL line), but since the upgrade the
performance had dropped to around 20KB/s!
Strangely enough, I could open many new connections to the guest and get
more chunks all at 20KB/s!
I switched the guests to using ne2k_pci and the performance has been
restored...

And this is where it gets even weirder...
UDP packets get corrupted using ne2k_pci and rtl8139cp but not with
virtio...
So I can get performance or UDP, but not both...

Let me know if there is anything more I can provide to help fix this
regression.
I can reproduce the problem quite easily without causing problems on the
host.

Cheers
Antoine