Messages in this thread | | | Date | Wed, 22 Aug 2007 12:22:17 +0200 | From | Andi Kleen <> | Subject | Re: [PATCH] Add I/O hypercalls for i386 paravirt |
| |
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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |