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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Xen-Devel (E-mail)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] RE: TSC scaling and softtsc reprise, and PROPOSAL
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Tue, 28 Jul 2009 12:10:05 -0700 (PDT)
Cc: "Dong, Eddie" <eddie.dong@xxxxxxxxx>, John Levon <levon@xxxxxxxxxxxxxxxxx>
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> >> I can live with that, if it is driven from the xend
> >> toolstack. It will have
> >> to default off in the hypervisor for compatibility with old
> >> saved images.
> > 
> > Hmmm... one could argue that with the current model,
> > any VM using TSC is "at your own peril" and there are
> > certainly cases of restore that will break whatever
> > assumptions the VM is making about pre-save TSC
> > values.   So while I'm a believer in compatibility,
> > I'd suggest default ON in the hypervisor and
> > a new restore option that force-overrides the
> > softtsc boot-time default for any VM being restored.
> It would be defaulted on by the toolstack for all newly 
> created guests.
> That's quite sufficient I think.

I guess I'm concerned that there are many toolstacks
that will need to be fixed, but there is one hypervisor.
Defaulting to softtsc in the hypervisor essentially
fixes the problem for the future and makes it clear
that the Xen developers have made a decision; waiting
for various vendor toolstacks to enforce a default (not
to mention going through the argument to convince
each vendor) presents a mixed message, prolongs the agony,
and almost guarantees chaos for years to come.

This is a subtle but fundamental change in the way
Xen works, necessary for correctness.  I think we
should bite the bullet and do it right.

Can the hypervisor itself tell the difference whether
a domain is being created vs restored?  I think not,
but if it can, that might be a good compromise.


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