From Proxmox VE

Introduction

In order to guarantee migration between physical hosts does not result in non-functioning virtual machines, QEMU & KVM disable the guest's ability to directly access some of the features which may be exclusive to the host CPU.

In the case where machine migration does not occur, ie. in machines 'assigned' to a particular node (due to hardware restrictions, etc.), this behavior can be bypassed, and may provide significant performance benefits.

Instructions

Editing the config file

1. Add the following line to the end of the applicable guest machine's configuration file, which is located at /etc/qemu-server/VMID.conf *

* Where the letters 'VMID' are substituted for the Machine number, ie. 101, 102, etc.

args: -cpu host

2. Save the file.

3. Stop then start the VM (a reboot is not enough to initiate the change).

Related Links

'CPU Performance' on the official KVM project website: http://www.linux-kvm.org/page/Tuning_KVM