centos虚拟机里面为何执行ethtool不显示速率？

最近蛮多类似询问，虚拟机里面执行ethtool为何不显示速率，但是windows却能显示速率为10Gbps。上图上图：

与物理主机不同，在kvm虚拟化环境中，ethtool仅返回了网口状态。来看看红帽的说明：

Issue

• How do I check the network speed of avirtio-netnetwork interface?
• When a VM use virtio NICs,ethtoolis not working
• Why virtio network nic speed is 1G in subnetwork tab in RHEV Manager web interface ?

Resolution

• This is expected, there is no set speed that avirtio-netinterface can operate at.
• The 1Gbps is just a placeholder because the driver has to report something.

Root Cause

• There's no "set speed" that avirtio-netinterface obtains. It's a software implementation of a network interface,it obtains the fastest possible speed it can, and that speed will vary depending on the network/CPU/memory load on the virtualization host at the time, as well the traffic.

• Consider that the emulatede1000network interfaces will report their speed as 1Gbps because they are a software emulation of a 1GbE interface. However, if you do traffic between two VMs on the same host using the emulatede1000interfaces, you will likely find you can exceed 1Gbps without a problem.If you emulate thise1000interface on a physical host with a 100Mbps network connection, then your guest will still say it has a 1-gigabit interface, but the maximum throughput attainable is 10% of that or less.Through all this,ethtoolwill still report the interface is operating at 1Gbps.

• The behavior of thevirtio-netinterfaces is the same, except the performance will be better because it's para-virtualized instead of emulated, and it won't report speed inethtool.

• There is no way theethtoolprocess within a guest can possibly know the maximum speed that a givenvirtio-netinterface can obtain at any given time, so there is nothing forvirtio-netto report toethtool.

总结下，在kvm环境，如果使用virtio_net驱动，linux下不会显示速率，会尽可能使用物理网络性能；如果配置的是e1000网卡，则会输出速率为1Gbps，但这个显示不代表实际能达到的速率，比如物理网络是100M，在e1000场景，也还是会显示1G。类似的，在windows下。默认不支持virtio驱动，需要安装驱动后才能支持，其速率也是驱动层面设置，而非实际的物理网络速率。