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

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Am 05.12.2016 um 17:02 schrieb Radim Krčmář:
> 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.

Okay I see. So if existing user space/guests don't break, there is no 
reason to make it configurable. I was just not sure if user space might 
want to decide whether to act "the old way".

>
>>                      (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.
>

Wonder if this is actually a bugfix for allowing KVM_MAX_VCPU_ID to
be > 255. Currently it is somewhat like

"yes, I support VCPU ids with > 255, but no, you can't really hotplug
such CPUs".

(fix for older kernels would then be limiting the VCPU ID to 255 and
not introducing a new capability).

But I am no expert on that topic, so feel free to ignore.

The general idea of this patch makes sense to me (x2apic hack)!

-- 

David

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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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