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

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Message-ID: <4D716E43.4080909@redhat.com>
Date:	Fri, 04 Mar 2011 17:57:07 -0500
From:	Zachary Amsden <zamsden@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	Nikola Ciprich <extmaillist@...uxbox.cz>,
	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>
Subject: Re: regression - 2.6.36 -> 2.6.37 - kvm - 32bit SMP guests don't
 boot

On 03/04/2011 02:09 PM, Glauber Costa wrote:
> On Fri, 2011-03-04 at 19:27 +0100, Nikola Ciprich wrote:
>    
>> Hello Zachary,
>>
>>      
>>> You don't see any messages about TSC being unstable or switching
>>> clocksource after loading the KVM module?  And you are not suspending
>>> the host or anything?
>>>        
>> no messages, no suspending, nothing.
>>
>>
>>      
>>> Can you try using "processor.max_cstate=1" on the host as a kernel
>>> parameter and see if it makes a difference?
>>>        
>> I tried it, no change..
>> n.
>>      
> Zach,
>
> I don't understand 100 % the logic behind all your tsc changes.
> But kvm-clock-wise, most of the problems we had in the past were related
> to the difference in resolution between the tsc and the host clocksource
> (hpet, acpi_pm, etc), which in his case, it is a non-issue.
>
> It does seem to me like some compensation logic kicked in, dismantling
> an otherwise good tsc. He does have nonstop_tsc, which means it can't
> get any better.
>
> One thing I noticed when reading the culprit patch in bisect, is that in
> vcpu_load(), there were previously a call to
>
>   kvm_request_guest_time_update(vcpu)
>
> that was removed without a counterpart addition. Any idea about why it
> was done?
>    

That's probably the source of the bug... I've been looking for that 
exact line, though, and I can't find it missing.
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