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

Avi Kivity
2007-08-22T05:34:36Z

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.
>

Won't these workloads be better off using paravirtualized drivers?
i.e., do the native drivers with paravirt I/O instructions get anywhere
near the performance of paravirt drivers?

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.