Re: [Xen-devel] Xen, Windows 2003 and APIC

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 05/12/2008 14:03, "Gary Pennington" <Gary.Pennington@xxxxxxx> wrote:

> My next question is: What is really happening when APIC is specified for
> a windows guest and why does performance vary so much according to whether
> it's specified or not?

Older Windows kernels update the APIC TPR a lot, and unless you have a very
modern Intel processor every one of those TPR updates causes a vmexit.

Modern Windows (including possibly latest w2k3 service pack, but I'm not
totally certain) includes lazy TPR, which gets rid of the vast majority of
TPR updates, and hence will go much faster.

 -- Keir



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