Re: [Xen-users] SOLVED - Poor Windows 2003 + GPLPV performance compared to VMWare

[James Harper <james.harper@xxxxxxxxxxxxxxxx>]

> > Microsoft updated Windows 2003 with SP2 to remove the use of the TPR
> > register, which was a big performance hit on a virtual system. You can add
> > /PATCHTPR to boot.ini to patch the windows kernel to use an alternate
> > method of TPR access under AMD systems, and to cache reads of TPR under
> > Intel systems, but I thought the xen guys had implemented their own
> > acceleration of this in xen itself?
> 
> Are you saying the SP1 with the /PATCHTPR will work well (not that I'm
> interested in this, but just interested), or are you saying the with SP2
> you can use /PATCHTPR to improve performance even further (this would be
> interesting)?
> 

The former. The TPR register is what Windows uses to manage interrupt 
priorities, and is accessed many many times per second. Every access causes a 
VMEXIT, so a whole lot of work gets done every time it is accessed. /PATCHTPR 
modifies the windows kernel a bit to change the way this is accessed.

With SP2, Microsoft modified the kernel to manage interrupt priorities a 
different way so the TPR register is not used at all, which virtualises much 
better.

/PATCHTPR is still useful for XP and 2000 which don't have these optimisations 
though.

James

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