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RE: [Xen-devel] [PATCH] replace rdtsc emulation-vs-native xen boot optio

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: RE: [Xen-devel] [PATCH] replace rdtsc emulation-vs-native xen boot option with per-domain (hypervisor part)
From: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>
Date: Tue, 29 Sep 2009 01:27:00 +0100
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Cc: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] [PATCH] replace rdtsc emulation-vs-native xen boot option with per-domain (hypervisor part)
> > Well, it shouldn't be enabled by default.  That slows down all rdtsc
> > operations for the benefit of very niche applications.  The Xen
> > clocksource assumes that rdtsc is fast, unemulated and in need of
> > correction.
> >
> > If someone really needs an artificial tsc, then they can enable the
> > option for themselves.
> 
> While I understand and sympathize (and the idealistic side of me even
> agrees with) your position, let me argue your point "they can enable
> the option for themselves" by mangling a famous movie quote:
> 
>       "In a cloud, no-one can hear rdtsc scream"

If we had some actual bug reports detailing failures of real-world applications 
then the decision would be easy.

I realise it could be hard to pin strange application behaviour down to TSC 
issues, but I can't readily spot any bugs on the XenServer or xen.org bug 
trackers that might even be candidates. 

Ian


 

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