[libvirt] why is network i/o always zero?

hi all I'm using Xen on CentOS.I wanna monitor the network activity of a domain. When I use Domain.InterfaceStats( ... ), which is the C# binding of virDomainInterfaceStats, it returns -1. I wonder if the function is not supported by Xen. So I using "domifstat", and I get vif1.0 rx_bytes 0 vif1.0 rx_packets 0 vif1.0 rx_errs 0 vif1.0 rx_drop 0 vif1.0 tx_bytes 0 vif1.0 tx_packets 0 vif1.0 tx_errs 0 vif1.0 tx_drop 0 but it never change even when I use that domain to ping another. virsh version: Compiled against library: libvir 0.8.4 Using library: libvir 0.8.4 Using API: Xen 3.0.1 Running hypervisor: Xen 3.3.0 My question is, is Xen not support virDomainInterfaceStats or is somewhere I've gone wrong, that I can't get the network i/o statistics? Thank you ! 2011-05-11 黄亮