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

Avi Kivity
2007-08-22T08:37:46Z

Zachary Amsden wrote:
> Avi Kivity 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.
>>>
>>>
>>
>>
>> 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?
>>
>
> Yes, in general, this is true (better off with paravirt drivers).
> However, we have "paravirt" drivers which run in both
> fully-paravirtualized and fully traditionally virtualized
> environments. As a result, they use native port I/O operations to
> interact with virtual hardware.

Suffering from terminology overdose here: "fully traditionally
virtualized, fully-paravirtuallized, para-fullyvirtualized".

Since this is only for newer kernels, won't updating the driver to use a
hypercall be more efficient? Or is this for existing out-of-tree drivers?

>
> Since not all hypervisors have paravirtualized driver infrastructures
> and guest O/S support yet, these hypercalls can be advantages to a
> wide range of scenarios. Using I/O hypercalls as such gives exactly
> the same performance as paravirt drivers for us, by eliminating the
> costly decode path, and the simplicity of using the same driver code
> makes this a huge win in code complexity.

Ah, seems the answer to the last question is yes.

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