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

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Message-ID: <b87fb1d9-643e-9c19-8732-8eb43afbf226@redhat.com>
Date:   Mon, 5 Dec 2016 15:37:37 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     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

Am 02.12.2016 um 20:44 schrieb Radim Krčmář:
> 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.

Should we allow user space to turn it on/off for compatibility handling? 
Or do we just not care? (or how will this capability be used later on?)



-- 

David