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

[Zhang, Yang Z]

Paolo Bonzini wrote on 2015-04-24:
> 
> 
> On 24/04/2015 09:46, Zhang, Yang Z wrote:
>>> 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.
> 
> I was going from memory, but I now measured it with the vmexit test of
> kvm-unit-tests.  With both SNB Xeon E5 and IVB Core i7, returns about
> 1400 clock cycles for a vmcall exit.  This includes the overhead of
> doing the cpuid itself.
> 
> Thus the vmexit cost is around 1300 cycles.  Of this the vmresume
> instruction is probably around 800 cycles, and the rest is introduced
> by KVM.  There are at least 4-5 memory barriers and locked instructions.

Yes, that's make sense. The average vmexit/vmentry handle cost is around 4000 cycles. But I guess xsaveopt doesn't take so many cycles. Does anyone have the xsaveopt cost data?

> 
> Paolo


Best regards,
Yang


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