linux-kernel - 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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Date:   Thu, 3 Mar 2022 19:49:05 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org
Subject: Re: [PATCH 4/4] KVM: x86: lapic: don't allow to set non default apic
 id when not using x2apic api

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".