linux-kernel - Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff

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Date:   Tue, 6 Dec 2016 13:52:31 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Igor Mammedov <imammedo@...hat.com>
Subject: Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over
 0xff

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.