linux-kernel - Re: [PATCH 0/6] KVM support for TSC scaling

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Message-ID: <4D6392F1.1030601@redhat.com>
Date:	Tue, 22 Feb 2011 12:41:53 +0200
From:	Avi Kivity <avi@...hat.com>
To:	"Roedel, Joerg" <Joerg.Roedel@....com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 0/6] KVM support for TSC scaling

On 02/22/2011 12:35 PM, Roedel, Joerg wrote:
> >  This doesn't really work, since we don't know on what host the TSC
> >  calibration loop ran:
> >
> >  - start guest on host H1
> >  - migrate it around, now it's on host H2
> >  - guest reboots, reruns calibration loop
> >  - migrate it around some more, now it's on host H3
> >  - migrate to host with tsc multiplier Hnew
> >
> >  So, what should we set the multiplier to? H1, H2, or H3's tsc rate?
>
> This scenario doesn't matter. If the guest already detected its TSC to
> be unstable there is nothing we can do and it doesn't really matter what
> we set the tsc frequency to. Therefore software will always set the
> guest tsc frequency to the same value it had on the last host.

Ok, so your scenario is

- boot on host H1
- no intervening migrations
- migrate to host Hnew
- all succeeding migrations are only to new hosts or back to H1

This is somewhat artificial, and not very different from an all-new cluster.

[the whole thing is kind of sad; we went through a huge effort to make 
clocks work on virtual machines in spite of the tsc issues; then we have 
a hardware solution, but can't use it because of old hardware.  Same 
thing happens with the effort put into shadow in the pre-npt days]

-- 
error compiling committee.c: too many arguments to function

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