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

H. Peter Anvin
2007-08-22T06:00:47Z

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.
> 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.
>
> This patch provides hypercalls for the i386 port I/O instructions, which
> vastly helps guests which use native-style drivers. For certain VMI
> workloads, this provides a performance boost of up to 30%. We expect
> KVM and lguest to be able to achieve similar gains on I/O intensive
> workloads.
>

What about cost on hardware?

-hpa