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

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Date:	Tue, 22 Feb 2011 12:11:42 +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 02/21/2011 07:28 PM, Roedel, Joerg wrote:
> >  - what's the cost of wrmsr(TSC_MULT)?
>
> Hard to tell by now because I only have numbers for pre-production
> hardware.

Can you ask your hardware people what the cost will likely be?  msrs are 
often expensive, and here we have two in the lightweight exit path.

> >  There are really two ways to implement this feature.  One is fully
> >  generic, like you did.  The other is to implement it at the host level -
> >  have a sysfs file and/or kernel parameter for the desired tsc frequency,
> >  write it once, and forget about it.  Trust management to set the host
> >  tsc frequency to the same value on all hosts in a migration cluster.
>
> The motivation here is mostly the flexibility. Scale the TSC for the
> whole migration cluster only makes sense if all hosts there support the
> feature. But the most likely scenario is that existing migration
> clusters will be extended by new machines and guests will be migrated
> there. And these guests should be able to see the same TSC frequency on
> the new host as the had on the old one. The older machines in the
> cluster may even have different TSC frequencys. With this flexible
> implementation those scenarios are possible. A host-wide setting for the
> scaling will make the feature useless in those (common) scenarios.

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?

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

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