Re: [PATCH] Add I/O hypercalls for i386 paravirt

Andi Kleen
2007-08-22T09:28:36Z

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