Xen devel - Mar 2007 - [Patch] Add VMX memory-mapped Local APIC access optimization

Some operating systems access the local APIC TPR very frequently, and we
handle that using software-based local APIC virtualization in Xen today.
Such virtualization incurs a number of VM exits from the memory-access
instructions against the APIC page in the guest. 

The attached patch enables the TPR shadow feature that provides APIC TPR
virtualization in hardware. Our tests indicate it can significantly
boost the performance of such guests including 32-bit Windows XP/2003. 

Moreover, with the patch, local APIC accesses other than TPR in guests
are intercepted directly as APIC_ACCESS VM exits rather than PAGE_FAULT
VM exits; this can lower the emulation cost of such accesses.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>



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Is there documentation describing this VMX extension? And what about
availability of silicon?

 -- Keir

On 21/3/07 02:08, "Cui, Dexuan" <dexuan.cui@intel.com> wrote:
> Some operating systems access the local APIC TPR very frequently, and we
> handle that using software-based local APIC virtualization in Xen today.
> Such virtualization incurs a number of VM exits from the memory-access
> instructions against the APIC page in the guest.
> 
> The attached patch enables the TPR shadow feature that provides APIC TPR
> virtualization in hardware. Our tests indicate it can significantly
> boost the performance of such guests including 32-bit Windows XP/2003.
> 
> Moreover, with the patch, local APIC accesses other than TPR in guests
> are intercepted directly as APIC_ACCESS VM exits rather than PAGE_FAULT
> VM exits; this can lower the emulation cost of such accesses.
> 
> Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>
> 


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