Re: [PATCH 4/4] KVM: x86: lapic: don't allow to set non default apic id when not using x2apic api

[Sean Christopherson]

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".