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

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Date:   Wed, 7 Dec 2016 16:47:16 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Igor Mammedov <imammedo@...hat.com>
Subject: Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over
 0xff

2016-12-07 13:03+0100, Paolo Bonzini:
> On 02/12/2016 20:44, Radim Krčmář wrote:
>> LAPIC after reset is in xAPIC mode, which poses a problem for hotplug of
>> VCPUs with high APIC ID, because reset VCPU is waiting for INIT/SIPI,
>> but there is no way to uniquely address it using xAPIC.
>> 
>> From many possible options, we chose the one that also works on real
>> hardware: accepting interrupts addressed to LAPIC's x2APIC ID even in
>> xAPIC mode.
>> 
>> KVM intentionally differs from real hardware, because real hardware
>> (Knights Landing) does just "x2apic_id & 0xff" to decide whether to
>> accept the interrupt in xAPIC mode and it can deliver one interrupt to
>> more than one physical destination, e.g. 0x123 to 0x123 and 0x23.
>> 
>> Add a capability to let userspace know that we do something now.
> 
> I wouldn't even bother with the capability.  It's a bit borderline for
> stable, but I think it's okay.  We can always Cc and let the maintainer
> reject it.

Ok, David also mentioned that I should tone down compatibility ...
Thanks to both, I'll prepare v2.