Messages in this thread | | | Subject | Re: [PATCH 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff | From | David Hildenbrand <> | 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
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