Messages in this thread | | | Date | Tue, 6 Dec 2016 13:52:31 +0100 | From | Radim Krčmář <> | Subject | Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff |
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2016-12-06 10:37+0100, David Hildenbrand: >> 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 > :) )
Just half a year old, since v4.7.
> 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.
This fix is too big for stable and I don't think that patches outside of stable get backported much.
> (especially as hotplugging cpuid > 255 doesn't sound like setups wildly used > already today - and it doesn't work ;) ).
Yes, it seems that no-one using high APIC ID noticed/cared.
> But as I said, I don't know the > history, so you decide if this check in QEMU is necessary.
QEMU can decide not to check (I actually expect it won't :]). I think the option to check is worth two lines of code in KVM, though.
> Fix all QEMUs (introduce capability check) vs fix all relevant kernels > (limiting VCPU id to 255).
APID ID over 255 works without hotplug and has few users, so lowering the limit would regress cases that are more important, IMO.
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