Virtual machine migration greatly aids management by allowing flexible provisioning of resources and decommissioning of hardware for maintenance. However, efforts to improve network performance by granting virtual machines direct access to hardware currently prevent migration. This occurs because (1) theVMMcannot migrate the state of the device, and (2) the source and destination machines may have different network devices, requiring different drivers to run in the migrated virtual machine.

In this paper, we describe a lightweight software mechanism for migrating virtual machines with direct hardware access. We base our solution on shadow drivers, an agent in the guest OS kernel that efficiently captures and restores the state of a device driver. On the source machine, the shadow driver monitors the state of the driver and device. After migration, the shadow driver uses this information to configure a driver for the corresponding device on the destination machine. We implement shadow driver migration for Linux network drivers running on the Xen hypervisor. Shadow driver migration requires a migration downtime similar to the driver initialization time, short enough to avoid disrupting active TCP connections. We find that the performance overhead, compared to direct hardware access, is negligible and is much lower than using a virtual NIC.