From: Zachary Amsden <zamsden@redhat.com>
To: Julien Desfossez <ju@klipix.org>
Cc: kvm@vger.kernel.org
Subject: Re: vread in kvm_clock
Date: Fri, 17 Dec 2010 07:43:53 -1000 [thread overview]
Message-ID: <4D0BA159.5050400@redhat.com> (raw)
In-Reply-To: <4D092225.3000300@klipix.org>
On 12/15/2010 10:16 AM, Julien Desfossez wrote:
> Hi,
>
> I'm currently working with the kvm clocksource and I'm wondering if we
> could implement the vread function for this clock source when we are
> running on a host with constant_tsc.
> If I understand correctly the hv_clock structure is per_cpu because of
> the eventual frequency changes, but in the case of constant_tsc (and
> after validation that the TSC is synchronized across all the cores) I
> think we could have a working vread function.
>
> In case of migration, could we have a fallback in case we detect we
> end up on a CPU without constant_tsc ?
>
> Any advice/explanation would be greatly appreciated !
It's a bit more complex than that. In addition to the problem you
mention with migration, even if the TSC is synchronized, the kvmclock
still is not, even with constant_tsc. There is measurement error in
between reading the TSC and computing the per_cpu hv_clock offset which
varies between CPUs.
Zach
next prev parent reply other threads:[~2010-12-17 17:43 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-12-15 20:16 vread in kvm_clock Julien Desfossez
2010-12-17 17:43 ` Zachary Amsden [this message]
2010-12-19 15:27 ` Avi Kivity
2010-12-20 1:15 ` Zachary Amsden
2010-12-20 6:16 ` Avi Kivity
2010-12-20 20:48 ` Zachary Amsden
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