linux-kernel - Re: [PATCH] allow userspace to adjust kvmclock offset

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Date:	Thu, 15 Oct 2009 11:58:56 -0300
From:	Glauber Costa <glommer@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] allow userspace to adjust kvmclock offset

On Thu, Oct 15, 2009 at 09:46:52AM +0900, Avi Kivity wrote:
> On 10/13/2009 09:46 PM, Glauber Costa wrote:
>> On Tue, Oct 13, 2009 at 03:31:08PM +0300, Avi Kivity wrote:
>>    
>>> On 10/13/2009 03:28 PM, Glauber Costa wrote:
>>>      
>>>>        
>>>>> Do we want an absolute or relative adjustment?
>>>>>
>>>>>          
>>>> What exactly do you mean?
>>>>
>>>>        
>>> Absolute adjustment: clock = t
>>> Relative adjustment: clock += t
>>>      
>> The delta is absolute, but the adjustment in the clock is relative.
>>
>> So we pick the difference between what userspace is passing us and what
>> we currently have, then relatively adds up so we can make sure we won't
>> go back or suffer a too big skew.
>>    
>
> The motivation for relative adjustment is when you have a jitter  
> resistant place to gather timing information (like the kernel, which can  
> disable interrupts and preemption), then pass it on to kvm without  
> losing information due to scheduling.  For migration there is no such  
> place since it involves two hosts, but it makes sense to support  
> relative adjustments.
Since we added the padding you asked for, we could use that bit of information
to define whether it will be a relative or absolute adjustment, then. Right now,
I don't see the point of implementing a code path that will be completely untested.

I'd leave it this way until someone comes up with a need.
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