Re: [RFC PATCH v2 2/2] add support for Hyper-V invariant TSC

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, May 23, 2013 at 12:12:29PM +0300, Gleb Natapov wrote:
> > To address migration scenarios to physical platforms that do not support
> > iTSC, the TscSequence field is used. In the event that a guest partition
> > is  migrated from an iTSC capable host to a non-iTSC capable host, the
> > hypervisor sets TscSequence to the special value of 0xFFFFFFFF, which
> > directs the guest operating system to fall back to a different clock
> > source (for example, the virtual PM timer)."
> > 
> > Why it would not/does not work after migration?
> > 
> Please read the whole discussion, we talked about it already. We
> definitely do not want to fall back to PM timer either, we want to use
> reference counter instead.

Case 1) On migration of TSC page enabled Windows guest, from invariant TSC host,
to non-invariant TSC host, Windows guests fallback to PMTimer
and not to reference timer via MSR. 

This is suboptimal because pmtimer emulation is excessively slow.

Is there a better option?

Case 2)
Reference timer (via MSR) support is interesting for the case of non invariant TSC
host.