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

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.


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