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

[Zachary Amsden]

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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