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When we initially built the infrastructure, it was still homogeneous—all CPUs were Sandy Bridge.  We configured Nova/libvirt so that the native features of the Hypervisor host were exposed to instances.  This had the advantage that VMs were able to use all of the functionality (e.g. new vector or crypto instructions) that the CPUs provided.

Since Because we store VM's block devices (virtual disks) are all stored in a shared Ceph cluster via OpenStack's RBD (RADOS Block Device) mapping, we have always been able to migrate instances from one hypervisor to another without stopping them ("live migration").  This is extremely useful for doing maintenance on the hypervisors, e.g. when security patches need to be applied to the base OS or to the virtual machine manager (QEMU/KVM).

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