Currently in Xen, when live migration is possible (i.e nomigrate=0); we don’t expose the invariant TSC flags, which makes the guest to rely on the PV clock instead. However, for timer-sensitive workloads (observed with Redis), it comes with a very large performance impact. Moreover, not having a reliable TSC in the guest can cause issues with software/OS that wants to rely on it anyway.
Modern CPUs now provide ways to assist in virtualization of the TSC (TSC offset and scaling) for dealing with that; but the Xen/XAPI/xl integration is lackluster and doesn’t work very well with it.
We want to improve the Xen side of that, such as we can (reasonably) reliably expose it to guests while still allowing live migration.