linux-kernel - Re: [kvm-devel] [PATCH] raise tsc clocksource rating

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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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