linux-kernel - Re: [PATCH] KVM: halt-polling: poll if emulated lapic timer will fire soon

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Date:	Thu, 19 May 2016 20:00:28 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH] KVM: halt-polling: poll if emulated lapic timer will fire soon

2016-05-19 19:56 GMT+08:00 Christian Borntraeger <borntraeger@...ibm.com>:
> On 05/19/2016 01:48 PM, Wanpeng Li wrote:
>> 2016-05-19 19:42 GMT+08:00 Christian Borntraeger <borntraeger@...ibm.com>:
>>> On 05/19/2016 01:35 PM, Wanpeng Li wrote:
>>>> 2016-05-19 19:23 GMT+08:00 Christian Borntraeger <borntraeger@...ibm.com>:
>>>>> On 05/19/2016 11:26 AM, Wanpeng Li wrote:
>>>>>
>>>>> I think in general a good idea to poll if a timer will expire soon.
>>>>>
>>>>> Some patch comments:
>>>>>
>>>>> Same for all non-x86 archs:
>>>>>> +static inline unsigned int kvm_arch_timer_remaining(struct kvm_vcpu *vcpu) {}
>>>>>
>>>>> A function returning int, without a return statement?
>>>>> That gives at least a compiler warning.
>>>>
>>>> How about return 0 for all non-x86 archs?
>>>
>>> We will provide an s390 implementation soon, but until then a proper
>>> default would be good.
>>>
>>> [....]
>>>>>> +     if (vcpu->halt_poll_ns || (remaining < halt_poll_ns_base)) {
>>>
>>> but then remaining is 0 and the 2nd condition will always be true, no?
>>
>> Nice catch!
>>
>> How about something like below:
>>
>> +       if (vcpu->halt_poll_ns ||
>> +               (remaining != 0 && remaining < halt_poll_ns_base)) {
>
> Maybe just use -1UL to have a "will never expire" and change the return value into u64
> while changing that.

Good idea, I will do it in next version.

Regards,
Wanpeng Li