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

[Zhang, Yang Z]

Paolo Bonzini wrote on 2015-04-24:
> 
> 
> 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).
> 

1000 cycles? I remember it takes about 4000 cycle even in HSW server.

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


Best regards,
Yang


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