linux-kernel - Re: [Hypervisors] TSC frequency change

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Message-ID: <48E34741.1030608@redhat.com>
Date:	Wed, 01 Oct 2008 11:47:45 +0200
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	akataria@...are.com
CC:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"avi@...hat.com" <avi@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Zach Amsden <zach@...are.com>,
	Daniel Hecht <dhecht@...are.com>,
	"Jun.Nakajima@...el.Com" <Jun.Nakajima@...el.Com>
Subject: Re: [Hypervisors] TSC frequency change

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
> 
> 

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