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>
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: <d05489c5-323d-505a-dbb9-1281b84947d0@redhat.com>
Date: Mon, 5 Dec 2016 19:00:11 +0100
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Am 05.12.2016 um 17:02 schrieb Radim Krčmář:
> 2016-12-05 15:37+0100, David Hildenbrand:
>> 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?
>
> There should be no guest that relies on the previous behavior, so I'd
> forgo the toggle, because it would be extra conditions in the code.
> I'd add it as a flag to KVM_CAP_X2APIC_API if you have reasons to let
> userspace choose.

Okay I see. So if existing user space/guests don't break, there is no 
reason to make it configurable. I was just not sure if user space might 
want to decide whether to act "the old way".

>
>>                      (or how will this capability be used later on?)
>
> New userspace should check this capability and disable hotplug of VCPUs
> with id over 255 if KVM doesn't support it.
>

Wonder if this is actually a bugfix for allowing KVM_MAX_VCPU_ID to
be > 255. Currently it is somewhat like

"yes, I support VCPU ids with > 255, but no, you can't really hotplug
such CPUs".

(fix for older kernels would then be limiting the VCPU ID to 255 and
not introducing a new capability).

But I am no expert on that topic, so feel free to ignore.

The general idea of this patch makes sense to me (x2apic hack)!

-- 

David