Re: [PATCH RFC 1/2] kvm vcpu: Note down pause loop exit

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 07/11/2012 02:23 PM, Avi Kivity wrote:
> On 07/09/2012 09:20 AM, Raghavendra K T wrote:
>> Signed-off-by: Raghavendra K T<raghavendra.kt@linux.vnet.ibm.com>
>>
>> Noting pause loop exited vcpu helps in filtering right candidate to yield.
>> Yielding to same vcpu may result in more wastage of cpu.
>>
>>
>>   struct kvm_lpage_info {
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index f75af40..a492f5d 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -3264,6 +3264,7 @@ static int interrupt_window_interception(struct vcpu_svm *svm)
>>
>>   static int pause_interception(struct vcpu_svm *svm)
>>   {
>> +	svm->vcpu.arch.plo.pause_loop_exited = true;
>>   	kvm_vcpu_on_spin(&(svm->vcpu));
>>   	return 1;
>>   }
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 32eb588..600fb3c 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -4945,6 +4945,7 @@ out:
>>   static int handle_pause(struct kvm_vcpu *vcpu)
>>   {
>>   	skip_emulated_instruction(vcpu);
>> +	vcpu->arch.plo.pause_loop_exited = true;
>>   	kvm_vcpu_on_spin(vcpu);
>>
>
> This code is duplicated.  Should we move it to kvm_vcpu_on_spin?
>
> That means the .plo structure needs to be in common code, but that's not
> too bad perhaps.
>

Since PLE is very much tied to x86, and proposed changes are very much 
specific to PLE handler, I thought it is better to make arch specific.

So do you think it is good to move inside vcpu_on_spin and make ple
structure belong to common code?

>> index be6d549..07dbd14 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -5331,7 +5331,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>
>>   	if (req_immediate_exit)
>>   		smp_send_reschedule(vcpu->cpu);
>> -
>> +	vcpu->arch.plo.pause_loop_exited = false;
>
> This adds some tiny overhead to vcpu entry.  You could remove it by
> using the vcpu->requests mechanism to clear the flag, since
> vcpu->requests is already checked on every entry.

So IIUC,  let's have request bit for indicating PLE,

pause_interception() /handle_pause()
{
  make_request(PLE_REQUEST)
  vcpu_on_spin()

}

check_eligibility()
  {
  !test_request(PLE_REQUEST) || ( test_request(PLE_REQUEST)  && 
dy_eligible())
.
.
}

vcpu_run()
{

check_request(PLE_REQUEST)
.
.
}

Is this is the expected flow you had in mind?

[ But my only concern was not resetting for cases where we do not do 
guest_enter(). will test how that goes].

>
>>   	kvm_guest_enter();
>>