Subject: Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over
 0xff
To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
References: <20161202194401.10038-1-rkrcmar@redhat.com>
 <20161202194401.10038-5-rkrcmar@redhat.com>
 <b87fb1d9-643e-9c19-8732-8eb43afbf226@redhat.com>
 <20161205160228.GA8660@potion>
 <d05489c5-323d-505a-dbb9-1281b84947d0@redhat.com>
 <20161205205728.GB7972@potion>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Igor Mammedov <imammedo@redhat.com>
From: David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <6f14273e-8429-18dc-6785-5f001b194bf1@redhat.com>
Date: Tue, 6 Dec 2016 10:37:03 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
 Thunderbird/45.4.0
MIME-Version: 1.0
In-Reply-To: <20161205205728.GB7972@potion>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 861
Lines: 22


> 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