From: "Radim Krčmář" <rkrcmar@redhat.com>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Paolo Bonzini <pbonzini@redhat.com>,
Igor Mammedov <imammedo@redhat.com>
Subject: Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff
Date: Tue, 6 Dec 2016 13:52:31 +0100 [thread overview]
Message-ID: <20161206125231.GC7972@potion> (raw)
In-Reply-To: <6f14273e-8429-18dc-6785-5f001b194bf1@redhat.com>
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.
next prev parent reply other threads:[~2016-12-06 12:53 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-02 19:43 [PATCH 0/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff Radim Krčmář
2016-12-02 19:43 ` [PATCH 1/4] KVM: x86: use delivery to self in hyperv synic Radim Krčmář
2016-12-05 14:41 ` David Hildenbrand
2016-12-05 16:03 ` Radim Krčmář
2016-12-02 19:43 ` [PATCH 2/4] KVM: x86: replace kvm_apic_id with kvm_{x,x2}apic_id Radim Krčmář
2016-12-02 19:44 ` [PATCH 3/4] KVM: x86: make interrupt delivery fast and slow path behave the same Radim Krčmář
2016-12-02 19:44 ` [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff Radim Krčmář
2016-12-05 14:37 ` David Hildenbrand
2016-12-05 16:02 ` Radim Krčmář
2016-12-05 18:00 ` David Hildenbrand
2016-12-05 20:57 ` Radim Krčmář
2016-12-06 9:37 ` David Hildenbrand
2016-12-06 12:52 ` Radim Krčmář [this message]
2016-12-07 12:03 ` Paolo Bonzini
2016-12-07 15:47 ` Radim Krčmář
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