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).
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