linux-kernel - Re: [PATCH 1/2] keep guest wallclock in sync with host clock

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Message-ID: <20090902124839.GM30340@mothafucka.localdomain>
Date:	Wed, 2 Sep 2009 09:48:39 -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 1/2] keep guest wallclock in sync with host clock

On Wed, Sep 02, 2009 at 03:24:26PM +0300, Avi Kivity wrote:
> On 09/02/2009 03:21 PM, Glauber Costa wrote:
>>
>>>> +static void kvm_sync_wall_clock(struct work_struct *work)
>>>> +{
>>>> +	struct timespec now;
>>>> +
>>>> +	kvm_get_wall_ts(&now);
>>>>
>>>>        
>>> What happens if we schedule here?
>>>      
>> hummm, I guess disabling preemption would be enough to make us safe here?
>>    
>
> You can't prevent host preemption.  You might read kvmclock again and  
> repeat if too much time has passed.
But then you can be scheduled after you did settimeofday, but before reading
kvmclock again. Since we're aiming for periodic adjustments here,
any discrepancies should not last long, so we can maybe live with it.

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