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RE: [Xen-devel] write_tsc in a PV domain?

To: Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] write_tsc in a PV domain?
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Fri, 28 Aug 2009 10:49:55 -0700 (PDT)
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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> > Jeremy's claim is that because some apps-that-use-
> > rdtsc risk bugginess, Xen can claim rdtsc for its own
> > use and effectively disallow all uses of rdtsc in any
> > app by breaking the existing, sometimes-useful semantics
> > of the instruction.
> If Xen is hiding the tsc cpu feature from the
> kernel/apps it can.

True, it can, but Xen does not currently do so and there
has been no proposal for Xen to do so.  And given Xen's
policy of supporting all existing applications, I don't
expect that a proposal to hide the tsc cpu feature
will fly.

> One problem there is a lot of grotty code simply
> explodes without rdtsc working.

Indeed.  While it might be satisfying to legislate
against stupidity, it rarely works. :-)

> The alternative is to virtualise the TSC as some other 
> hypedvisors do but that has other impacts.

Yes, this is where this whole discussion started.
Let me summarize, but start a separate thread to do so.

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