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Message-ID: <4726B218.8010101@qumranet.com>
Date: Tue, 30 Oct 2007 06:24:56 +0200
From: Avi Kivity <avi@...ranet.com>
To: Dan Hecht <dhecht@...are.com>
CC: Ingo Molnar <mingo@...e.hu>, Zachary Amsden <zach@...are.com>,
jeremy@...p.org, avi@...amnet.com, kvm-devel@...ts.sourceforge.net,
linux-kernel@...r.kernel.org,
Glauber de Oliveira Costa <gcosta@...hat.com>,
Glauber de Oliveira Costa <glauber@...ranet.com>,
--cc@...hat.com, Garrett Smith <garrett@...are.com>
Subject: Re: [kvm-devel] [PATCH] raise tsc clocksource rating
Dan Hecht wrote:
> Not really. In the case hardware TSC is perfect, the paravirt time
> counter can be implemented directly in terms of hardware TSC; there is
> no loss in optimization. This is done transparently. And virtual TSC
> can be implemented this way too.
>
> The real improvement that a paravirt clocksource offers over the TSC
> clocksource is that the guest does not need to measure the TSC frequency
> itself against some other constant frequency source (which is
> problematic on a virtual machine). Instead, the paravirt clocksource
> queries the hypervisor for the frequency of the counter. As you know,
> with clocksource style kernels, it's important to get this frequency
> correct, or else the guest will have long-term time drift.
>
>
In addition, a paravirt clocksource can compensate for events like vcpu
migration to another host cpu. So I agree: a paravirt clocksource is
always better than or equal to the tsc.
--
Any sufficiently difficult bug is indistinguishable from a feature.
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