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
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