Re: [RFC] KVM: optimize the kvm_vcpu_on_spin

[Paolo Bonzini]

On 01/08/2017 05:26, Longpeng (Mike) wrote:
> 
> 
> On 2017/7/31 21:20, Paolo Bonzini wrote:
> 
>> On 31/07/2017 14:27, David Hildenbrand wrote:
>>>> I'm not sure whether the operation of get the vcpu's priority-level is
>>>> expensive on all architectures, so I record it in kvm_sched_out() for
>>>> minimal the extra cycles cost in kvm_vcpu_on_spin().
>>>>
>>> as you only care for x86 right now either way, you can directly optimize
>>> here for the good (here: x86) case (keeping changes and therefore
>>> possible bugs minimal).
>>
>> I agree with Cornelia that this is inconsistent, so you shouldn't update
>> me->in_kernmode in kvm_vcpu_on_spin.  However, get_cpl requires
>> vcpu_load on Intel x86, so Mike's patch is necessary (vmx_get_cpl ->
>> vmx_read_guest_seg_ar -> vmcs_read32).
>>
> 
> Hi Paolo,
> 
> It seems that other architectures(e.g. arm/s390) needn't to cache the result,
> but x86 need, so I need to move 'in_kernmode' into kvm_vcpu_arch and only add
> this field to x86, right?

That's another way to do it, and I like it.

>>  This will cache the result until the next sched_in, so that
> 
> 'until the next sched_in' --> Do we need to clear the result in sched in ?

No, thanks.

Paolo