Hi,

currently we go through the emulator every time a HVM guest does an I/O 
port access (in/out). This is unnecessary most of the times, as both VMX 
and SVM provide all the necessary information already in the VMCS/VMCB. 
String instructions are not covered by this shortcut, but they are quite 
rare and we would need to access the guest memory anyway.
This patch decodes the information from VMCB/VMCS and calls a simple 
handle_mmio wrapper. In handle_mmio() itself the emulation part will 
simply be skipped, this approach avoids code duplication.
Since the vendor specific part is quite trivial, I implemented both the 
VMX and SVM part, please check the VMX part for sanity.

I boot-tested both versions and ran some simple benchmarks.
A micro benchmark (hammering an I/O port in a tight loop) shows a 
significant performance improvement (down to 66% of the time needed to 
handle the intercept on an AMD K8, measured in the guest with TSC).
Even with reading a 1GB file from an emulated IDE harddisk (Dom0 cached) 
I could get a  4-5% improvement.
Some guest code (e.g. the TCP stack in some Windows version) exercises 
the PM-Timer I/O port (0x1F48) very often (multiple 10,000 times per 
second), these workloads also benefit with up to 5% improvement from 
this patch.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>

Regards,
Andre.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12