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Date:   Thu, 3 Mar 2022 19:49:05 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Maxim Levitsky <mlevitsk@redhat.com>
Cc:     kvm@vger.kernel.org, Jim Mattson <jmattson@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Borislav Petkov <bp@alien8.de>, x86@kernel.org
Subject: Re: [PATCH 4/4] KVM: x86: lapic: don't allow to set non default apic
 id when not using x2apic api
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References: <20220301135526.136554-1-mlevitsk@redhat.com>
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On Thu, Mar 03, 2022, Sean Christopherson wrote:
> With your proposed change, KVM_SET_LAPIC will fail and we've broken a functional,
> if sketchy, setup.  Is there likely to be such a real-world setup that doesn't
> barf on the inconsistent x2APIC ID?  Probably not, but I don't see any reason to
> find out.

I take back the "probably not", this isn't even all that contrived.  Prior to the
"migration", the guest will see a consistent x2APIC ID.  It's not hard to imagine
a guest that snapshots the ID and never re-reads the value from "hardware".