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RE: [Xen-users] XP SP3 guest high CPU usage

from [James Harper]

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Subject:	RE: [Xen-users] XP SP3 guest high CPU usage
From:	"James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date:	Tue, 19 May 2009 12:41:49 +1000
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Thread-topic:	[Xen-users] XP SP3 guest high CPU usage

> 
> Hi I'm getting very high CPU usage on idle for an XP SP3 guest, which
is doing
> nothing and is reporting 0% CPU within the guest.
> 

Make a copy of the default entry in your boot.ini and add /PATCHTPR to
it.

The TPR register is accessed very very frequently under XP (and 2003 <
sp2) and causes a VMEXIT (breakout to the hypervisor) every time it is
accessed. /PATCHTPR makes GPLPV patch the kernel to make it a bit
faster.

With an AMD CPU, /PATCHTPR will improve things greatly by using an
instruction that is only supported by AMD processors in 32 bit mode.

If you are using an Intel CPU, it will improve things a bit (my patch
caches the TPR register and only writes cause a slowdown).

Newer Intel CPU's and a newish version of Xen shouldn't have this
problem at all as Xen emulates access itself.

Let me know how it works out.

Thanks

James



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