2016-12-07 12:37+0100, David Hildenbrand: >> Yes, that is reasonable -- I though David wanted to use the feature when >> running on CPUs that support it (only mask off on the one). >> >> I'd start with changing the check in vmx_check_processor_compat() to >> ignore disabled features and then extend KVM to enable only the common >> set. >> >> Finding the minimal set of common features is complicated by hotplug ... >> I think that the check we have is not working perfectly, so if you >> currently >> >> 1) offline all cores on the worse CPU >> 2) load kvm module >> 3) online all CPUs >> >> then KVM is going enable tsc-scaling and not complain, but guests >> running on onlined CPUs are not going to work. >> Can you confirm that? > > My intuition tells me that whenever we hotplug CPUs that have less features > than used, we are in trouble. So tsc scaling might just be the tip of the > iceberg. I think this is a general problem. Definitely. It was not handled in KVM probably because it doesn't have a simple solution. Finding the minimal common subset on hotplug will take extra work, which is why relaxing the check and having a toggle for features that shouldn't be enabled nor checked is easier. > What should happen if we hotplug such CPUs? We can't run existing VCPUs on > them. And isn't this even a general problem, also for other tasks in the > system (how is that problem solved with cpuid features?)? 1) Prevent the hotplug -- admin can notice the error, kill guests or decide to let them finish, and then online hotplugged CPU. 2) Just warn and trust that admin knows what hotplugging to non-SMP means. (2) is less work and give a bit more freedom to an undesirable case, so I slightly prefer it. I wouldn't for example limit existing VCPUs to compatible CPUs or cleanly kill all guests from the kernel. > (I am currently thinking about "virsh capabilities", could it happen that > our "host" cpu model is no longer valid after we hotplugged cpus (as the > common feature set got smaller)? that would be very ugly) I think it could, but I'd continue in thinking only about SMP. VMX features are not even noticed by `virsh capabilities`, so finding the minimal common subset in KVM would not affect the output. -- 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
