linux-kernel - Re: regression - 2.6.36 -> 2.6.37 - kvm

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Date:	Thu, 03 Mar 2011 15:47:25 -0500
From:	Zachary Amsden <zamsden@...hat.com>
To:	Nikola Ciprich <extmaillist@...uxbox.cz>
CC:	Avi Kivity <avi@...hat.com>,
	Nikola Ciprich <nikola.ciprich@...uxbox.cz>,
	KVM list <kvm@...r.kernel.org>,
	Linux kernel list <linux-kernel@...r.kernel.org>,
	Glauber Costa <glommer@...hat.com>
Subject: Re: regression - 2.6.36 -> 2.6.37 - kvm - 32bit SMP guests don't
 boot

On 03/03/2011 02:06 AM, Nikola Ciprich wrote:
>> No worries.  What mess?
>>      
> twice sending the same mail, nevermind :)
>
>    
>> I have two things you can try:
>>
>> first is running a single VCPU guest, if you have not done so already.
>>      
> yup, UP guest is fine, just SMP doesn't work.
>
>    
>> Second is adding the bootparameter "clocksource=acpi_pm" to your guest
>> kernel.
>>      
> yes, this makes SMP work too! I just realized when You were asking about current
> clocksource, I told You only host source, not the guest. So I checked now,
> and (at least for UP, I guess for SMP it's the same), the clocksource is
> kvm-clock! So seems like it got broken with the TSC changes?
>    

What is the exact kernel version you are using in the guest.

It appears that some earlier 32-bit versions of kvm-clock enabled 
kernels are still missing the required atomic check for backwards-time 
protection which would be needed on SMP.  This explains why 64-bit is 
fine, 32-bit is not.

Why this change triggers that problem still is a slight mystery, 
logically it should only affect the system if you have an unstable TSC.

Zach
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