PCI passthrough

VT-d提供了将一个硬件设备（如网卡）直接分配给某个虚拟机，分配之后的I/O操作不需要hypervisor介入，因此可以获得非常高的性能，几乎和物理机上的IO吞吐量一样。该功能需要CPU和主板的支持，CPU需要支持VT-d，主板需要有IOMMU。直通的缺点硬件资源独用，且影响live migration。

更多信息参考：

http://blog.csdn.net/defeattroy/article/details/8746704

SR-IOV

The next steps in I/O virtualization are actually happening today. For example, PCIe includes support for virtualization. One virtualization concept that's ideal for server virtualization is called Single-Root I/O Virtualization (SR-IOV). This virtualization technology (created through the PCI-Special Interest Group, or PCI-SIG) provides device virtualization in single-root complex instances (in this case, a single server with multiple VMs sharing a device). Another variation, called Multi-Root IOV, supports larger topologies (such as blade servers, where multiple servers can access one or more PCIe devices). In a sense, this permits arbitrarily large networks of devices, including servers, end devices, and switches (complete with device discovery and packet routing).

  http://www.ibm.com/developerworks/library/l-pci-passthrough/

Redhat关于SR-IOV的howto

https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/5/html/Virtualization/sect-Para-virtualized_Windows_Drivers_Guide-How_SR_IOV_Libvirt_Works.html

http://docs.fedoraproject.org/en-US/Fedora/13/html/Virtualization_Guide/sect-Para-virtualized_Windows_Drivers_Guide-How_SR_IOV_Libvirt_Works.html