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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

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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