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

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Date:	Wed, 11 Jul 2012 17:26:15 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	S390 <linux-s390@...r.kernel.org>,
	Carsten Otte <cotte@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	KVM <kvm@...r.kernel.org>, chegu vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>, linux390@...ibm.com,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Joerg Roedel <joerg.roedel@....com>
Subject: Re: [PATCH RFC 1/2] kvm vcpu: Note down pause loop exit

On 07/11/2012 04:48 PM, Avi Kivity wrote:
> On 07/11/2012 01:52 PM, Raghavendra K T wrote:
>> 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@...ux.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?
>
> See the discussion with Christian.  PLE is tied to x86, but cpu_relax()
> and facilities to trap it are not.

Yep.

>
>>>
>>> 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?
>
> Yes, something like that.

ok..

>
>>
>> [ But my only concern was not resetting for cases where we do not do
>> guest_enter(). will test how that goes].
>
> Hm, suppose we're the next-in-line for a ticket lock and exit due to
> PLE.  The lock holder completes and unlocks, which really assigns the
> lock to us.  So now we are the lock owner, yet we are marked as don't
> yield-to-us in the PLE code.

Yes.. off-topic but that is solved by kicked flag in PV spinlocks.

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