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

from [Keir Fraser] [Permanent Link][Original]
To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>, "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: Thu, 23 Jul 2009 16:45:39 +0100
Cc: "Dong, Eddie" <eddie.dong@xxxxxxxxx>, John Levon <levon@xxxxxxxxxxxxxxxxx>
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On 23/07/2009 16:18, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

>> pre-VT it wasn't possible to trap RDTSC, so this can't help PV guests.
> 
> For PV guests, CR4.TSD would always be set, generating
> a general protection fault for every rdtsc.  (Or perhaps
> I am missing some x86 architectural subtlety?  This is
> how it is done on ia64.)

Forgot about CR4.TSD. Of course it's not going to be a super fast path,
since #GP(0) will need to be demuxed by decoding the faulting instruction in
the hypervisor, via a not-so-short path.

 -- Keir



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