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Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff

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2016-12-05 15:37+0100, David Hildenbrand:
> Am 02.12.2016 um 20:44 schrieb Radim Krčmář:
>> LAPIC after reset is in xAPIC mode, which poses a problem for hotplug of
>> VCPUs with high APIC ID, because reset VCPU is waiting for INIT/SIPI,
>> but there is no way to uniquely address it using xAPIC.
>> 
>> From many possible options, we chose the one that also works on real
>> hardware: accepting interrupts addressed to LAPIC's x2APIC ID even in
>> xAPIC mode.
>> 
>> KVM intentionally differs from real hardware, because real hardware
>> (Knights Landing) does just "x2apic_id & 0xff" to decide whether to
>> accept the interrupt in xAPIC mode and it can deliver one interrupt to
>> more than one physical destination, e.g. 0x123 to 0x123 and 0x23.
>> 
>> Add a capability to let userspace know that we do something now.
> 
> Should we allow user space to turn it on/off for compatibility handling? Or
> do we just not care?

There should be no guest that relies on the previous behavior, so I'd
forgo the toggle, because it would be extra conditions in the code.
I'd add it as a flag to KVM_CAP_X2APIC_API if you have reasons to let
userspace choose.

>                      (or how will this capability be used later on?)

New userspace should check this capability and disable hotplug of VCPUs
with id over 255 if KVM doesn't support it.

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[PATCH 0/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff Radim Krčmář <rkrcmar@redhat.com> - 2016-12-02 20:50 +0100
  [PATCH 2/4] KVM: x86: replace kvm_apic_id with kvm_{x,x2}apic_id Radim Krčmář <rkrcmar@redhat.com> - 2016-12-02 20:50 +0100
  [PATCH 3/4] KVM: x86: make interrupt delivery fast and slow path behave the same Radim Krčmář <rkrcmar@redhat.com> - 2016-12-02 20:50 +0100
  [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff Radim Krčmář <rkrcmar@redhat.com> - 2016-12-02 20:50 +0100
    Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over  0xff David Hildenbrand <david@redhat.com> - 2016-12-05 15:40 +0100
      Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over  0xff Radim Krčmář <rkrcmar@redhat.com> - 2016-12-05 17:10 +0100
        Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over  0xff David Hildenbrand <david@redhat.com> - 2016-12-05 19:10 +0100
          Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over  0xff Radim Krčmář <rkrcmar@redhat.com> - 2016-12-05 22:00 +0100
            Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over  0xff David Hildenbrand <david@redhat.com> - 2016-12-06 10:50 +0100
              Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over  0xff Radim Krčmář <rkrcmar@redhat.com> - 2016-12-06 14:00 +0100
    Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over  0xff Paolo Bonzini <pbonzini@redhat.com> - 2016-12-07 13:10 +0100
      Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over  0xff Radim Krčmář <rkrcmar@redhat.com> - 2016-12-07 16:50 +0100

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