linux-kernel - Re: [PATCH 14/18] Fix a possible backwards warp of kvmclock

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Message-ID: <4C3E744C.7060402@redhat.com>
Date:	Wed, 14 Jul 2010 22:37:00 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	KVM <kvm@...r.kernel.org>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/18] Fix a possible backwards warp of kvmclock

On 07/12/2010 10:25 PM, 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).
>
> We must instead compute the clock as potentially observed by the guest
> for kernel_ns to make sure it does not go backwards.
>
> Signed-off-by: Zachary Amsden<zamsden@...hat.com>

Acked-by: Rik van Riel <riel@...hat.com>

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