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Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy

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On 18/02/2016 18:33, Paolo Bonzini wrote:
> On 18/02/2016 17:56, Radim Krčmář wrote:
>> 2016-02-18 17:13+0100, Paolo Bonzini:
>>> On 17/02/2016 20:14, Radim Krčmář wrote:
>>>> Discard policy uses ack_notifiers to prevent injection of PIT interrupts
>>>> before EOI from the last one.
>>>>
>>>> This patch changes the policy to always try to deliver the interrupt,
>>>> which makes a difference when its vector is in ISR.
>>>> Old implementation would drop the interrupt, but proposed one injects to
>>>> IRR, like real hardware would.
>>>
>>> This seems like what libvirt calls the "merge" policy:
>>
>> Oops, I never looked beyond QEMU after seeing that the naming in libvirt
>> doesn't even match ...
>>
>> I think the policy that KVM implements (which I call discard) is "delay"
>> in libvirt.  (https://libvirt.org/formatdomain.html#elementsTime)
> 
> Suppose the scheduled ticks are at times 0, 20, 40, 60, 80.  The EOI for
> time 0 is only delivered at time 42, other EOIs are timely.
> 
> The resulting injections are:
> 
> - for discard: 0, 60, 80.
> 
> - for catchup, which QEMU calls slew: 0, 42, 51, 60, 80.
> 
> - for merge: 0, 20 (in IRR, delivered at 42), 60, 80.
> 
> For delay I *think* it would be 0, 42, 62, 82, 102.

Wrong: for delay it is something like 0, 42, 43, 60, 80.

Your patch does the right thing, QEMU is wrong in calling the policy
"discard" where it should have been "merge".  In fact both i8254 and RTC
use the same wrong nomenclature.

And it is indeed superfluous to use ack notifiers to implement the
default policy, as the default policy is already baked into the i8259.

Sorry, it shows that I'm swamped as I'm messing up things a bit lately.
 At least I have you to correct me. :)

Paolo

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[PATCH v2 00/14] KVM: x86: change PIT discard policy and untangle related code Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack_irq Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
    Re: [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack_irq Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:10 +0100
      Re: [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack_irq Paolo Bonzini <pbonzini@redhat.com> - 2016-02-19 17:00 +0100
      Re: [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack_irq Radim Krčmář <rkrcmar@redhat.com> - 2016-02-19 17:00 +0100
  [PATCH v2 14/14] KVM: x86: move PIT timer function initialization Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 05/14] KVM: x86: tone down WARN_ON pit.state_lock Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 06/14] KVM: x86: pass struct kvm_pit instead of kvm in PIT Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 09/14] KVM: x86: refactor kvm_create_pit Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 04/14] KVM: x86: use atomic_t instead of pit.inject_lock Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 13/14] KVM: x86: don't assume layout of kvm_kpit_state Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
    Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 17:20 +0100
      Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-18 18:00 +0100
        Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 18:40 +0100
          Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:00 +0100
            Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-19 15:50 +0100
    Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:50 +0100
  [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
    Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard  policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:10 +0100
    Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard  policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:10 +0100
      Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard  policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-19 16:10 +0100
        Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard  policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-19 16:10 +0100
          Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard  policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-19 16:10 +0100
  [PATCH v2 10/14] KVM: x86: refactor kvm_free_pit Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 11/14] KVM: x86: remove pit and kvm from kvm_kpit_state Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 07/14] KVM: x86: remove unnecessary uses of PIT state lock Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
    Re: [PATCH v2 07/14] KVM: x86: remove unnecessary uses of PIT state  lock Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:10 +0100
      Re: [PATCH v2 07/14] KVM: x86: remove unnecessary uses of PIT state  lock Radim Krčmář <rkrcmar@redhat.com> - 2016-02-19 15:50 +0100
  [PATCH v2 03/14] KVM: x86: add kvm_pit_reset_reinject Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:30 +0100
  Re: [PATCH v2 00/14] KVM: x86: change PIT discard policy and untangle  related code Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:20 +0100

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