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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 11:15:38 -0700 (PDT)
Cc: "Dong, Eddie" <eddie.dong@xxxxxxxxx>, John Levon <levon@xxxxxxxxxxxxxxxxx>
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> > Savvy system admins (who know every app in their
> > data center and/or are willing to take the risk
> > for better performance) should be able to easily
> > disable softtsc though on all servers with a xen boot
> > option, or on a per VM basis.
> > 
> > We could quibble about details (maybe softtsc
> > should only be automatically enabled on SMP guests
> > or on 64-bit SMP guests or ?? ) but I suspect
> > that just creates a mess and IMHO we should just
> > bite the bullet.
> 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.
>  -- Keir

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.

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