On 2016年04月27日 00:17, Jan Kiszka wrote: > On 2016-04-26 18:14, Lan, Tianyu wrote: >> Hi All: >> >> Recently I am working on extending max vcpu to more than 256 on the both >> KVM/Xen. For some HPC cases, it needs many vcpus. The job requires to >> use X2APIC in the guest which supports 32-bit APIC id. Linux kernel >> requires irq remapping function during enabling X2APIC when max APIC id >> is more than 255(More detail please see try_to_enable_x2apic()). >> >> The irq remapping function helps to deliver irq to cpu 255~. IOAPIC just >> supports 8-bit target APIC id field and only can deliver irq to >> cpu 0~255. >> >> So far both KVM/Xen doesn't enable irq remapping function. If enable the >> function, it seems a huge job which need to rework IO-APIC, local APIC, >> MSI parts and add virtual VTD support in the KVM. >> >> Other quick way to enable more than 256 VCPUs is to eliminate the >> dependency between irq remapping and X2APIC in the guest linux kernel. >> So far I can boot the guest after removing the dependency. >> The side effect I thought is that irq only can deliver to 0~255 vcpus >> but 256 vcpus seem enough to balance irq requests in the guest. In the >> most cases, there are fewer devices in the guest. >> >> I wonder whether it's feasible. There maybe some other side effects I >> didn't think of. Very appreciate for your comments. > > Radim is working on the KVM side already, Peter is currently driving the > VT-d interrupt emulation topic in QEMU. It's in reach, I would say. :) Oh. Thanks for your information. Very helpful :) > > Jan > > PS: Please no PV mess, at least without good reasons. > -- Best regards Tianyu Lan -- 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
