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

[David Hildenbrand]

Am 05.12.2016 um 17:02 schrieb Radim KrÄmÃÅ:
> 2016-12-05 15:37+0100, David Hildenbrand:
> > Am 02.12.2016 um 20:44 schrieb Radim KrÄmÃÅ:
> > > 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