linux-kernel - Re: [KVM timekeeping 05/35] Move TSC reset out of vmcb

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Date:	Mon, 23 Aug 2010 14:52:41 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [KVM timekeeping 05/35] Move TSC reset out of vmcb_init

On 08/20/2010 07:08 AM, Glauber Costa wrote:
> On Thu, Aug 19, 2010 at 10:07:19PM -1000, Zachary Amsden wrote:
>    
>> The VMCB is reset whenever we receive a startup IPI, so Linux is setting
>> TSC back to zero happens very late in the boot process and destabilizing
>> the TSC.  Instead, just set TSC to zero once at VCPU creation time.
>>
>> Why the separate patch?  So git-bisect is your friend.
>>
>> Signed-off-by: Zachary Amsden<zamsden@...hat.com>
>>      
> Shouldn't we set for whatever value the BSP already has, and then the BSP to
> zero? Since vcpus are initialized at different times, this pretty much
> guarantees that the guest will have desynchronized tsc at all cases
> (not that if it was better before...)
>    

Yes, we should - but it takes a lot more machinery to do that, and so it 
happens later in the series.  You have to match the offsets for the BSP 
and other vcpus...
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