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Re: [PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API

From Paolo Bonzini <pbonzini@redhat.com>
Newsgroups linux.kernel
Subject Re: [PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API
Date 2016-07-11 09:50 +0200
Message-ID <rTE8y-42b-13@gated-at.bofh.it> (permalink)
References <rSl7X-1t6-7@gated-at.bofh.it> <rSl7X-1t6-25@gated-at.bofh.it> <rTCzL-3d1-7@gated-at.bofh.it>
Organization linux.* mail to news gateway

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On 11/07/2016 08:06, Yang Zhang wrote:
>> Changes to MSI addresses follow the format used by interrupt remapping
>> unit.
>> The upper address word, that used to be 0, contains upper 24 bits of
>> the LAPIC
>> address in its upper 24 bits.  Lower 8 bits are reserved as 0.
>> Using the upper address word is not backward-compatible either as we
>> didn't
>> check that userspace zeroed the word.  Reserved bits are still not
>> explicitly
> 
> Does this means we cannot migrate the VM from KVM_CAP_X2APIC_API enabled
> host to the disable host even VM doesn't have more than 255 VCPUs?

Yes, but that's why KVM_CAP_X2APIC_API is enabled manually.  The idea is
that QEMU will not use KVM_CAP_X2APIC_API except on the newest machine type.

If interrupt remapping is on, KVM_CAP_X2APIC_API is needed even with 8
VCPUs, I think.  Otherwise KVM will believe that 0xff is "broadcast"
rather than "cluster 0, CPUs 0-7".

Paolo

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Thread

[PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API Radim Krčmář <rkrcmar@redhat.com> - 2016-07-07 19:20 +0200
  Re: [PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API Yang Zhang <yang.zhang.wz@gmail.com> - 2016-07-11 08:10 +0200
    Re: [PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API Paolo Bonzini <pbonzini@redhat.com> - 2016-07-11 09:50 +0200
      Re: [PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API Yang Zhang <yang.zhang.wz@gmail.com> - 2016-07-11 11:00 +0200
        Re: [PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API Paolo Bonzini <pbonzini@redhat.com> - 2016-07-11 11:20 +0200
          Re: [PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API Yang Zhang <yang.zhang.wz@gmail.com> - 2016-07-11 12:40 +0200
            Re: [PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API Paolo Bonzini <pbonzini@redhat.com> - 2016-07-11 12:50 +0200

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