[Xen-users] PCI passthrough, poor network performance in upload direction

["Luis F Urrea" <lfurrea@xxxxxxxxx>]

Hi all,

I have been having an issue for over a year with several Xen installations and have not been able to find out if this is just something I am missing.

Basically I am trying to configure a firewall with two ethernet cards in a domU using pci passthrough. I am hiding the Ethernet cards at boot time via the pciback.hide kernel option.

Right now I am running Xen 3.3.0 and the two Ethernet cards are passed to the FW domU. 

The FW domU is not configured with a vif interface to the bridge, so basically communication with DomO passes through the external hardware.

Tests using iperf show me that download bandwidth from the FW domU to an external machine connected to the switch go as high as 94 Mbits/sec. However upload bandwidth from a laptop to the domU give me a maximum of 574 Kbits/sec which basically is not enough to cover the upload bw of some internet connections.

I have read recommendations about using ethtool -K ethX tx off in dom U, but in this case the tx ring does not seem the be the issue since the download speeds are just fine. Using 'ethtool -K rx eth0 off' did not show any improvements.

Since the guest domain has exclusive access to both Ethernet cards I suppose that the overhead is introduced somewhere in the domU's kernel but I wouldn't exactly know where else to look for a possible fix.

Your input is appreciated.

TIA

Luis
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