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

[Zachary Amsden <zach@vmware.com>]

H. Peter Anvin wrote:
> 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?
>   

On modern hardware, port I/O is about the most expensive thing you can 
do.  The extra function call cost is totally masked by the stall.  We 
have measured with port I/O converted like this on real hardware, and 
have seen zero measurable impact on macro-benchmarks.  Micro-benchmarks 
that generate massively repeated port I/O might show some effect on 
ancient hardware, but I can't even imagine a workload which does such a 
thing, other than a polling port I/O loop perhaps - which would not be 
performance critical in any case I can reasonably imagine.

Zach