linux-kernel - Re: [PATCH 6/5] KVM: x86: fix periodic lapic timer with hrtimers

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Date:   Mon, 24 Oct 2016 17:09:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Yunhong Jiang <yunhong.jiang@...el.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH 6/5] KVM: x86: fix periodic lapic timer with hrtimers



On 24/10/2016 17:03, Radim Krčmář wrote:
> I have only compile-tested it, but it should optimize the switch and
> also fix two bugs.  The first one is major.
> This needs that the deadline clearing in [5/5] is fixed.
> ---8<---
> We must start the hrimer even if the expiration is already in the past,
> otherwise the periodic timer would not rearm the hrtimer.
> 
> And computing next expiration of a period timer does not require current
> time.  The period should be constant, so it is more precise to add the
> period to the last expiration time.  This fixes a time difference
> between hrtimer expiration and apic->lapic_timer.target_expiration.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> ---
>  arch/x86/kvm/lapic.c | 38 +++++++++++++-------------------------
>  1 file changed, 13 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index f4734fe12803..6244988418be 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1350,19 +1350,19 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
>  
>  static void start_sw_period(struct kvm_lapic *apic)
>  {
> -	ktime_t now;
> -
> -	now = apic->lapic_timer.timer.base->get_time();
> -
>  	if (!apic->lapic_timer.period)
>  		return;
>  
> -	if (likely(ktime_after(apic->lapic_timer.target_expiration, now)))
> -		hrtimer_start(&apic->lapic_timer.timer,
> -			apic->lapic_timer.target_expiration,
> -			HRTIMER_MODE_ABS_PINNED);
> -	else
> +	if (apic_lvtt_oneshot(apic) &&
> +	    ktime_after(apic->lapic_timer.target_expiration,
> +	                apic->lapic_timer.timer.base->get_time())) {
>  		apic_timer_expired(apic);
> +		return;
> +	}
> +
> +	hrtimer_start(&apic->lapic_timer.timer,
> +		apic->lapic_timer.target_expiration,
> +		HRTIMER_MODE_ABS_PINNED);
>  }
>  
>  static bool set_target_expiration(struct kvm_lapic *apic)
> @@ -1414,10 +1414,11 @@ static bool set_target_expiration(struct kvm_lapic *apic)
>  
>  static void advance_periodic_target_expiration(struct kvm_lapic *apic)
>  {
> -	apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, rdtsc()) +
> +	apic->lapic_timer.tscdeadline +=
>  		nsec_to_cycles(apic->vcpu, apic->lapic_timer.period);
> -	apic->lapic_timer.target_expiration = ktime_add_ns(apic->lapic_timer.timer.base->get_time(),
> -		apic->lapic_timer.period);
> +	apic->lapic_timer.target_expiration =
> +		ktime_add_ns(apic->lapic_timer.target_expiration,
> +				apic->lapic_timer.period);
>  }
>  
>  bool kvm_lapic_hv_timer_in_use(struct kvm_vcpu *vcpu)
> @@ -1481,19 +1482,6 @@ void kvm_lapic_switch_to_hv_timer(struct kvm_vcpu *vcpu)
>  
>  	WARN_ON(apic->lapic_timer.hv_timer_in_use);
>  
> -	if (apic_lvtt_period(apic)) {
> -		ktime_t remaining, now;
> -		u64 tscl = rdtsc();
> -
> -		now = apic->lapic_timer.timer.base->get_time();
> -		remaining = hrtimer_get_remaining(&apic->lapic_timer.timer);
> -		if (ktime_to_ns(remaining) < 0)
> -			remaining = ktime_set(0, 0);
> -
> -		apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, tscl) +
> -			nsec_to_cycles(apic->vcpu, ktime_to_ns(remaining));
> -		apic->lapic_timer.target_expiration = ktime_add_ns(now, ktime_to_ns(remaining));
> -	}
>  	start_hv_timer(apic);
>  }
>  EXPORT_SYMBOL_GPL(kvm_lapic_switch_to_hv_timer);
> 

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>

Go ahead, squash it into 5/5 and commit to kvm/queue. :)

Paolo