Re: [PATCH] Fix almost infinite loop in APIC

[Alexander Graf <agraf@suse.de>]

Sheng Yang wrote:
> On Friday 09 January 2009 00:36:06 Alexander Graf wrote:
>   
>> While booting Linux in VMware ESX, I encountered a strange effect
>> in the in-kernel lapic implementation: time went backwards!
>>
>> While this should never occur, because of that the while loop that
>> is done after the invalid calculations caused my host system to hang.
>>
>> In order to make debugging easier, let's replace this as suggested
>> with a modulo function and not run into the danger of looping forever.
>>
>> To replace the nice hint this bug gave me that the values are broken,
>> I added a printk message so people encountering this can at least
>> see that something is fishy.
>>
>> Of course, the real issue needs to be fixed as well! I'm open to ideas
>> why now < last_update!
>>
>> (Thanks to Kevin for his help in debugging this)
>>
>> Signed-off-by: Alexander Graf <agraf@suse.de>
>> Signed-off-by: Kevin Wolf <kwolf@suse.de>
>> ---
>>     
>
> Hi Alexander
>
> I'm a little suspect here:
>
>   
>> 	if (unlikely(ktime_to_ns(now) <=
>> 		ktime_to_ns(apic->timer.last_update))) {
>> 		/* Wrap around */
>> 		passed = ktime_add(( {
>> 				    (ktime_t) {
>> 				    .tv64 = KTIME_MAX -
>> 				    (apic->timer.last_update).tv64}; }
>> 				   ), now);
>> 		apic_debug("time elapsed\n");
>> 	} else
>> 		passed = ktime_sub(now, apic->timer.last_update);
>>     
>
> And now apic timer base is hr_timer with CLOCK_MONOTONIC, and get_time() is 
> really ktime_get() which is almost impossible to wrap around. If it's 
> overflow, at least we need a warning. I think this piece of code due to clock 
> source change.
>
> So I doubt: due to some reason, now <= apic->timer.last_update, which cause a 
> big wrap around operation.
>
> And the most suspect:
>
>   
>> void kvm_apic_timer_intr_post(struct kvm_vcpu *vcpu, int vec)
>> {
>> 	struct kvm_lapic *apic = vcpu->arch.apic;
>>
>> 	if (apic && apic_lvt_vector(apic, APIC_LVTT) == vec)
>> 		apic->timer.last_update = ktime_add_ns(
>> 				apic->timer.last_update,
>> 				apic->timer.period);
>> }
>>     
>
> Not sure what's happening, have you tried this? (In fact, I am little willing 
> to replace all apic->timer.dev.base->get_time() with more explicit ktime_get() 
> as in pit.)
>   

Yes, this code is the culprit. Using that patch of yours now is always >
last_update. I'd still like to see that while loop go away ;-).

Alex

> --
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index afac68c..414e7e0 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1115,9 +1115,7 @@ void kvm_apic_timer_intr_post(struct kvm_vcpu *vcpu, int 
> vec)
>         struct kvm_lapic *apic = vcpu->arch.apic;
>
>         if (apic && apic_lvt_vector(apic, APIC_LVTT) == vec)
> -               apic->timer.last_update = ktime_add_ns(
> -                               apic->timer.last_update,
> -                               apic->timer.period);
> +               apic->timer.last_update = apic->timer.dev.base->get_time();
>  }
>
>  int kvm_get_apic_interrupt(struct kvm_vcpu *vcpu)
>
>
>