linux-kernel - Re: [PATCH v6 3/3] KVM: LAPIC: Apply change to TDCR right away to the timer

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Date:   Fri, 6 Oct 2017 15:14:54 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v6 3/3] KVM: LAPIC: Apply change to TDCR right away to
 the timer

2017-10-05 18:54-0700, Wanpeng Li:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> The description in the Intel SDM of how the divide configuration
> register is used: "The APIC timer frequency will be the processor's bus
> clock or core crystal clock frequency divided by the value specified in
> the divide configuration register."
> 
> Observation of baremetal shown that when the TDCR is change, the TMCCT
> does not change or make a big jump in value, but the rate at which it
> count down change.
> 
> The patch update the emulation to APIC timer to so that a change to the
> divide configuration would be reflected in the value of the counter and
> when the next interrupt is triggered.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> @@ -1458,6 +1458,36 @@ static void start_sw_period(struct kvm_lapic *apic)
>  		HRTIMER_MODE_ABS_PINNED);
>  }
>  
> +static bool update_target_expiration(struct kvm_lapic *apic, uint32_t old_divisor)
> +{
> +	ktime_t now, remaining;
> +	u64 tscl = rdtsc(), delta;
> +
> +	now = ktime_get();
> +	remaining = ktime_sub(apic->lapic_timer.target_expiration, now);
> +	if (ktime_to_ns(remaining) < 0)
> +		remaining = 0;
> +	delta = mod_64(ktime_to_ns(remaining), apic->lapic_timer.period);

Hm, can this happen?

> +	if (!delta)
> +		return false;
> +
> +	apic->lapic_timer.period = (u64)kvm_lapic_get_reg(apic, APIC_TMICT)
> +		* APIC_BUS_CYCLE_NS * apic->divide_count;

I think that it would be safer to always modify the period.

> +	delta = delta * apic->divide_count / old_divisor;
> +
> +	if (!apic->lapic_timer.period)
> +		return false;
> +
> +	limit_periodic_timer_frequency(apic);
> +
> +	apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, tscl) +
> +		nsec_to_cycles(apic->vcpu, delta);

We could do that without rdtsc() for added precision and maybe
performance:

	apic->lapic_timer.tscdeadline += nsec_to_cycles(apic->vcpu, delta) -
	                                 nsec_to_cycles(apic->vcpu, remaining);

	// not sure how a negative operand would behave:
	// nsec_to_cycles(apic->vcpu, delta - remaining)

> +	apic->lapic_timer.target_expiration = ktime_add_ns(now, delta);
> +
> +	return true;
> +}
> +
>  static bool set_target_expiration(struct kvm_lapic *apic)
>  {
>  	ktime_t now;
> @@ -1750,13 +1780,20 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
>  		start_apic_timer(apic);
>  		break;
>  
> -	case APIC_TDCR:
> +	case APIC_TDCR: {
> +		uint32_t old_divisor = apic->divide_count;
> +
>  		if (val & 4)
>  			apic_debug("KVM_WRITE:TDCR %x\n", val);
>  		kvm_lapic_set_reg(apic, APIC_TDCR, val);
>  		update_divide_count(apic);
> +		if (apic->divide_count != old_divisor) {
> +			hrtimer_cancel(&apic->lapic_timer.timer);
> +			if (update_target_expiration(apic, old_divisor))
> +				restart_apic_timer(apic);

I think we can lose a timer here when we cancel a hrtimer whose
expiration time passes before update_target_expiration(), so it never
gets restarted.

Doing restart_apic_timer() unconditionally seems better.  It behaves
well if we try to restart a timer that has already fired.

Thanks.

> +		}
>  		break;
> -
> +	}
>  	case APIC_ESR:
>  		if (apic_x2apic_mode(apic) && val != 0) {
>  			apic_debug("KVM_WRITE:ESR not zero %x\n", val);
> -- 
> 2.7.4
>