Re: [Hypervisors] TSC frequency change

[Gerd Hoffmann <kraxel@redhat.com>]

Alok Kataria wrote:
> Hi Gerd,
> 
> As Zach explained, we support a view that, tsc is constant.

Ok, so the guest doesn't have to worry about possible tsc changes when
using that interface.  Should go into the comment documenting the leaf.

> Coming back to the migration problem, as you too acknowledge, migration
> to a host with a different frequency should be seen as a different
> problem. I would be interested in learning about any proposal that you
> may have thought about to handle this.

xen paravirtualized is explained below.

xen full virtualized: dunno.

kvm: provide something else for timekeeping to avoid the tsc trouble
altogether if possible.  hpet, pm_timer, paravirtualized clocksource.
Obviously can't work for all guests though.  paravirtual clocksource
works like the xen one.

cheers,
  Gerd

>>> What special things does Xen do at migration, which would be affected by
>>> this interface ?
>> paravirtualized xen guests have a paravirtual clock.  That is a struct
>> containing three pieces of information: system time, tsc counter for the
>> last system time update, tsc frequency.  The guest gets the current time
>> by reading the system time and adding a delta calculated from current
>> tsc, tsc of last systime update and tsc frequency.  Handling tsc
>> frequency changes is obviously trivial here, just update the field on
>> the next systime update ;)
> 
> Oh nice, that is convenient. 
> 
> Thanks,
> Alok
> 
>