Subject: [PATCH] KVM: arm/arm64: Handle forward time correction gracefully

From: [email-masked] (Marc Zyngier)
Date: Wed, 6 Apr 2016 11:58:36 +0100

On 06/04/16 11:33, Christoffer Dall wrote:
> On Wed, Apr 06, 2016 at 09:37:22AM +0100, Marc Zyngier wrote:
>> On a host that runs NTP, corrections can have a direct impact on
>> the background timer that we program on the behalf of a vcpu.
>>
>> In particular, NTP performing a forward correction will result in
>> a timer expiring sooner than expected from a guest point of view.
>> Not a big deal, we kick the vcpu anyway.
>>
>> But on wake-up, the vcpu thread is going to perform a check to
>> find out whether or not it should block. And at that point, the
>> timer check is going to say "timer has not expired yet, go back
>> to sleep". This results in the timer event being lost forever.
>>
>> There are multiple ways to handle this. One would be record that
>> the timer has expired and let kvm_cpu_has_pending_timer return
>> true in that case, but that would be fairly invasive. Another is
>> to check for the "short sleep" condition in the hrtimer callback,
>> and restart the timer for the remaining time when the condition
>> is detected.
>>
>> This patch implements the latter, with a bit of refactoring in
>> order to avoid too much code duplication.
>>
>> Reported-by: Alexander Graf <[email-masked]>
>> Signed-off-by: Marc Zyngier <[email-masked]>
>> ---
>>  virt/kvm/arm/arch_timer.c | 47 +++++++++++++++++++++++++++++++++++++----------
>>  1 file changed, 37 insertions(+), 10 deletions(-)
>>
>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
>> index a9ad4fe..4d0e77a 100644
>> --- a/virt/kvm/arm/arch_timer.c
>> +++ b/virt/kvm/arm/arch_timer.c
>> @@ -98,10 +98,46 @@ static void kvm_timer_inject_irq_work(struct work_struct *work)
>>  	kvm_vcpu_kick(vcpu);
>>  }
>>  
>> +static u64 kvm_timer_compute_delta(struct kvm_vcpu *vcpu)
>> +{
>> +	cycle_t cval, now;
>> +
>> +	cval = vcpu->arch.timer_cpu.cntv_cval;
>> +	now = kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
>> +
>> +	if (now < cval) {
>> +		u64 ns;
>> +
>> +		ns = cyclecounter_cyc2ns(timecounter->cc,
>> +					 cval - now,
>> +					 timecounter->mask,
>> +					 &timecounter->frac);
>> +		return ns;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>>  static enum hrtimer_restart kvm_timer_expire(struct hrtimer *hrt)
>>  {
>>  	struct arch_timer_cpu *timer;
>> +	struct kvm_vcpu *vcpu;
>> +	u64 ns;
>> +
>>  	timer = container_of(hrt, struct arch_timer_cpu, timer);
>> +	vcpu = container_of(timer, struct kvm_vcpu, arch.timer_cpu);
>> +
>> +	/*
>> +	 * Check that the timer has really expired from the guest's
>> +	 * PoV (NTP on the host may have forced it to expire
>> +	 * early). If we should have slept longer, restart it.
>> +	 */
>> +	ns = kvm_timer_compute_delta(vcpu);
>> +	if (unlikely(ns)) {
>> +		hrtimer_forward_now(hrt, ns_to_ktime(ns));
>> +		return HRTIMER_RESTART;
>> +	}
>> +
>>  	queue_work(wqueue, &timer->expired);
>>  	return HRTIMER_NORESTART;
>>  }
>> @@ -176,8 +212,6 @@ static int kvm_timer_update_state(struct kvm_vcpu *vcpu)
>>  void kvm_timer_schedule(struct kvm_vcpu *vcpu)
>>  {
>>  	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
>> -	u64 ns;
>> -	cycle_t cval, now;
>>  
>>  	BUG_ON(timer_is_armed(timer));
>>  
>> @@ -197,14 +231,7 @@ void kvm_timer_schedule(struct kvm_vcpu *vcpu)
>>  		return;
>>  
>>  	/*  The timer has not yet expired, schedule a background timer */
>> -	cval = timer->cntv_cval;
>> -	now = kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
>> -
>> -	ns = cyclecounter_cyc2ns(timecounter->cc,
>> -				 cval - now,
>> -				 timecounter->mask,
>> -				 &timecounter->frac);
>> -	timer_arm(timer, ns);
>> +	timer_arm(timer, kvm_timer_compute_delta(vcpu));
>>  }
>>  
>>  void kvm_timer_unschedule(struct kvm_vcpu *vcpu)
>> -- 
>> 2.1.4
>>
> 
> How do you guys feel about adding this to the patch for improved sleep
> at night (pun intended):
> 
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index a9ad4fe..230f720 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -91,6 +91,8 @@ static void kvm_timer_inject_irq_work(struct work_struct *work)
>  	vcpu = container_of(work, struct kvm_vcpu, arch.timer_cpu.expired);
>  	vcpu->arch.timer_cpu.armed = false;
>  
> +	BUG_ON(!kvm_timer_should_fire(vcpu));
> +

I would probably turn this into a WARN_ON() rather than bringing the
whole system down (keeping it alive would help debugging), but otherwise
looks good to me.

>  	/*
>  	 * If the vcpu is blocked we want to wake it up so that it will see
>  	 * the timer has expired when entering the guest.
> 
> 
> Otherwise:
> 
> Reviewed-by: Christoffer Dall <[email-masked]>
> 
> And I can queue this with CC to stable, but I would like Alex's
> tested-by if possible.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...