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

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Message-ID: <20100616135845.GM19104@mothafucka.localdomain>
Date:	Wed, 16 Jun 2010 10:58:45 -0300
From:	Glauber Costa <glommer@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	Zachary Amsden <zamsden@...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 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?

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