linux-kernel - Re: [PATCH RFC 2/2] KVM: x86: Support using the VMX preemption timer for APIC Timer periodic/oneshot mode

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Date:   Wed, 12 Oct 2016 16:35:26 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Yunhong Jiang <yunhong.jiang@...el.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH RFC 2/2] KVM: x86: Support using the VMX preemption timer
 for APIC Timer periodic/oneshot mode

2016-10-11 23:06 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2016-10-11 20:17+0800, Wanpeng Li:
>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>
>> Most windows guests still utilize APIC Timer periodic/oneshot mode
>> instead of tsc-deadline mode, and the APIC Timer periodic/oneshot
>> mode are still emulated by high overhead hrtimer on host. This patch
>> converts the expected expire time of the periodic/oneshot mode to
>> guest deadline tsc in order to leverage VMX preemption timer logic
>> for APIC Timer tsc-deadline mode.
>>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Cc: Yunhong Jiang <yunhong.jiang@...el.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> ---
>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>> @@ -1101,13 +1101,20 @@ static u32 apic_get_tmcct(struct kvm_lapic *apic)
>>               apic->lapic_timer.period == 0)
>>               return 0;
>>
>> +     if (kvm_lapic_hv_timer_in_use(apic->vcpu)) {
>> +             u64 tscl = rdtsc();
>> +
>> +             tmcct = apic->lapic_timer.tscdeadline -
>> +                     kvm_read_l1_tsc(apic->vcpu, tscl);
>
> Yes, this won't work.  The easiest way to return a less bogus TMCCT
> would be remember the timeout when setting up the timer and replace
> hrtimer_get_remaining() with it -- our deliver method shouldn't change
> the expiration time.

Agreed.

>
>> +     } else {
>> +             remaining = hrtimer_get_remaining(&apic->lapic_timer.timer);
>> +             if (ktime_to_ns(remaining) < 0)
>> +                     remaining = ktime_set(0, 0);
>> +
>> +             ns = mod_64(ktime_to_ns(remaining), apic->lapic_timer.period);
>> +             tmcct = div64_u64(ns,
>> +                              (APIC_BUS_CYCLE_NS * apic->divide_count));
>> +     }
>>
>>       return tmcct;
>>  }
>> @@ -1400,52 +1407,65 @@ bool kvm_lapic_hv_timer_in_use(struct kvm_vcpu *vcpu)
>>  }
>>  EXPORT_SYMBOL_GPL(kvm_lapic_hv_timer_in_use);
>>
>> -static void cancel_hv_tscdeadline(struct kvm_lapic *apic)
>> +static void cancel_hv_timer(struct kvm_lapic *apic)
>>  {
>>       kvm_x86_ops->cancel_hv_timer(apic->vcpu);
>>       apic->lapic_timer.hv_timer_in_use = false;
>>  }
>>
>> +static bool start_hv_timer(struct kvm_lapic *apic)
>>  {
>> +     u64 tscdeadline;
>>
>> +     if (apic_lvtt_period(apic) || apic_lvtt_oneshot(apic)) {
>> +             u64 tscl = rdtsc();
>>
>> +             apic->lapic_timer.period = (u64)kvm_lapic_get_reg(apic, APIC_TMICT)
>> +                     * APIC_BUS_CYCLE_NS * apic->divide_count;
>> +             apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, tscl) +
>> +                     nsec_to_cycles(apic->vcpu, apic->lapic_timer.period);
>
> start_hv_timer() is called from kvm_lapic_switch_to_hv_timer(), which
> can happen mid-period.  The worst case is that the timer will never
> fire, because we always convert back and forth.  You have to compute the
> equivalent deadline only once, and carry it around.

Agreed. Thanks for your review. :) Please see RFC V2.

Regards,
Wanpeng Li

>
>> +     }
>> +
>> +     tscdeadline = apic->lapic_timer.tscdeadline;
>>
>>       if (atomic_read(&apic->lapic_timer.pending) ||
>>               kvm_x86_ops->set_hv_timer(apic->vcpu, tscdeadline)) {
>>               if (apic->lapic_timer.hv_timer_in_use)
>> +                     cancel_hv_timer(apic);
>>       } else {
>>               apic->lapic_timer.hv_timer_in_use = true;
>>               hrtimer_cancel(&apic->lapic_timer.timer);
>>
>>               /* In case the sw timer triggered in the window */
>>               if (atomic_read(&apic->lapic_timer.pending))
>> +                     cancel_hv_timer(apic);
>>       }
>>       trace_kvm_hv_timer_state(apic->vcpu->vcpu_id,
>>                       apic->lapic_timer.hv_timer_in_use);
>>       return apic->lapic_timer.hv_timer_in_use;
>>  }
>>
>> +void kvm_lapic_expired_hv_timer(struct kvm_vcpu *vcpu)
>> +{
>> +     struct kvm_lapic *apic = vcpu->arch.apic;
>> +
>> +     WARN_ON(!apic->lapic_timer.hv_timer_in_use);
>> +     WARN_ON(swait_active(&vcpu->wq));
>> +     cancel_hv_timer(apic);
>> +     apic_timer_expired(apic);
>> +
>> +     if (apic_lvtt_period(apic))
>> +             start_hv_timer(apic);
>> +}
>> +EXPORT_SYMBOL_GPL(kvm_lapic_expired_hv_timer);
>> +
>>  void kvm_lapic_switch_to_hv_timer(struct kvm_vcpu *vcpu)
>>  {
>>       struct kvm_lapic *apic = vcpu->arch.apic;
>>
>>       WARN_ON(apic->lapic_timer.hv_timer_in_use);
>>
>> -     if (apic_lvtt_tscdeadline(apic))
>> -             start_hv_tscdeadline(apic);
>> +     start_hv_timer(apic);
>>  }
>>  EXPORT_SYMBOL_GPL(kvm_lapic_switch_to_hv_timer);
>>