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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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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