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[Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel (E-mail)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 20 Jul 2009 18:14:46 +0100
Cc: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Dong, Eddie" <eddie.dong@xxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>, John Levon <levon@xxxxxxxxxxxxxxxxx>
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On 20/07/2009 18:05, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> The default mode for all xen systems should be that all rdtsc
> instructions should be emulated by xen using xen system time
> as the timestamp counter (i.e. nanosecond frequency).
> The no-softtsc Xen boot option remains available to force the
> non-trapping mechanism if desired.  It might make sense to
> add a per-guest config option to override per guest.
> The Xen CPU info emulation should reflect that tsc is constant
> and safe to use on an SMP.
> Comments?  I think someone at Intel (Eddie?) was studying the
> TSC emulation path to see if it could be faster, but I'm not
> sure where that ended up.

Defaults which slow things down are never popular. The slowdown on a
non-idle Solaris guest, for example, could be significant. It is a
correctness/accuracy vs performance tradeoff though. But I don't think there
are many real-world complaints about the TSC accuracy now -- I think the
default is set appropriately.

 -- Keir

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