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SubjectRe: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff
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> I think the agreement is to embrace compatibility, so we pile new
> mistakes to hide known ones.
> (Rewriting the past requires far more power than accepting it:
> If we didn't force unfixed kernels out of existence, then userspace
> couldn't tell if hotplug up to high VCPU ID limit is supported.)

I agree, the question is how old the bug is (you should know better than
me :) ) and if introducing a capability is strictly necessary. Do we
have to do the check in QEMU or can we simply fix implementations out
there silently.

(especially as hotplugging cpuid > 255 doesn't sound like setups wildly
used already today - and it doesn't work ;) ). But as I said, I don't
know the history, so you decide if this check in QEMU is necessary.

Fix all QEMUs (introduce capability check) vs fix all relevant kernels
(limiting VCPU id to 255).

--

David

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