Re: [v6] kvm/fpu: Enable fully eager restore kvm FPU

[Paolo Bonzini]

On 24/04/2015 03:16, Zhang, Yang Z wrote:
>> This is interesting since previous measurements on KVM have had
>> the exact opposite results.  I think we need to understand this a
>> lot more.
> 
> What I can tell is that vmexit is heavy. So it is reasonable to see
> the improvement under some cases, especially kernel is using eager
> FPU now which means each schedule may trigger a vmexit.

On the other hand vmexit is lighter and lighter on newer processors; a
Sandy Bridge has less than half the vmexit cost of a Core 2 (IIRC 1000
vs. 2500 clock cycles approximately).

Also, measurement were done on Westmere but Sandy Bridge is the first
processor to have XSAVEOPT and thus use eager FPU.

Paolo
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