On Tue, Aug 21, 2007 at 10:23:14PM -0700, Zachary Amsden wrote:

In general, I/O in a virtual guest is subject to performance problems.  
The I/O can not be completed physically, but must be virtualized.  This 
means trapping and decoding port I/O instructions from the guest OS.  
Not only is the trap for a #GP heavyweight, both in the processor and 
the hypervisor (which usually has a complex #GP path), but this forces 
the hypervisor to decode the individual instruction which has faulted.  

Is that really that expensive? Hard to imagine.

e.g. you could always have a fast check for inb/outb at the beginning
of the #GP handler. And is your initial #GP entry really more expensive
than a hypercall? 

Worse, even with hardware assist such as VT, the exit reason alone is 
not sufficient to determine the true nature of the faulting instruction, 
requiring a complex and costly instruction decode and simulation.

It's unclear to me why that should be that costly.

Worst case it's a switch()

-Andi