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Re: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Fri, 11 Dec 2009 08:03:11 +0000
Cc: "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>
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On 11/12/2009 02:00, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> I expect that Keir will not support putting TSC_AUX
> in the context switch code unless it is absolutely
> necessary, as it is certainly expensive to read and
> write to TSC_AUX and this cost will add to every
> context switch of every VM even though very few will
> actually use rdtscp/TSC_AUX.

Well, you'd make it dependent on the guest using TSC_AUX, I suppose. I think
that's going to be pretty rare.

> So I think we need to decide first about approach (1),
> the full faithful implementation of rdtscp.

The question has to be: what win do we get for faithful virtualisation of
RDTSCP in a virtualised environment? Supporting CPU instructions just
because they're there is not a useful effort.

 -- Keir



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