2016-12-05 19:00+0100, David Hildenbrand: > 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". I also don't see a reason for userspace to want it disabled -- it just shouldn't matter even if userspace implements another solution (e.g. it hotplugs VCPUs in x2APIC mode) or KVM ends up with a better solution. Any change can break some guest, but I couldn't with anything reasonable that would be broken. >> > (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 Good point, it is, for guests that want hotplug. I'll add Fixes: line; thanks! > "yes, I support VCPU ids with > 255, but no, you can't really hotplug > such CPUs". My bad, offline/online in Linux worked fine so I didn't think enough about hotplug. > (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. I think the agreement is to embrace compatibility, so we pile new mistakes to hide known ones. (Rewriting the past requires far more power than accepting it: If we didn't force unfixed kernels out of existence, then userspace couldn't tell if hotplug up to high VCPU ID limit is supported.) > The general idea of this patch makes sense to me (x2apic hack)! The situation would be a bit better if xAPIC ID was read-only (we'd behave more like real-hardware then), but no major OS changes the ID, which makes it a secondary concern with weird corner-cases. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
