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

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Message-ID: <573DA9D3.3070205@de.ibm.com>
Date:	Thu, 19 May 2016 13:56:03 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Wanpeng Li <kernellwp@...il.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

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.