linux-kernel - Re: [PATCH 11/17] Fix a possible backwards warp of kvmclock

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Date:	Wed, 16 Jun 2010 12:38:30 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	Jason Wang <jasowang@...hat.com>, avi@...hat.com,
	mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/17] Fix a possible backwards warp of kvmclock

On 06/16/2010 03:58 AM, Glauber Costa wrote:
> On Wed, Jun 16, 2010 at 04:11:26PM +0800, Jason Wang wrote:
>    
>> Zachary Amsden wrote:
>>      
>>> Kernel time, which advances in discrete steps may progress much slower
>>> than TSC.  As a result, when kvmclock is adjusted to a new base, the
>>> apparent time to the guest, which runs at a much higher, nsec scaled
>>> rate based on the current TSC, may have already been observed to have
>>> a larger value (kernel_ns + scaled tsc) than the value to which we are
>>> setting it (kernel_ns + 0).
>>>
>>>        
>> This is one issue of kvmclock which tries to supply a clocksource whose
>> precision may even higher than host.
>>      
> What if we export to the guest the current clock resolution, and when doing guest
> reads, simply chop whatever value we got to the lowest acceptable value?
>    

I considered it, but it still doesn't solve the problem, at least, not 
without adding TSC trap and emulate.  If you can see a higher resolution 
clock advance faster than the resolution of the kernel, you still have 
the problem, and any visible TSC will do that.
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