Re: Enable more than 255 VCPU support without irq remapping function in the guest

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-04-27 13:39+0800, Lan Tianyu:
> On 2016年04月27日 00:49, Radim Krčmář wrote:
>> 2016-04-26 18:17+0200, Jan Kiszka:
>>> 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()).
>> 
>> Our of curiosity, how many VCPUs are you aiming at?
> 
> I think it's 1024.
> 
> In the short term, we hope hypervisor at least supports 288 vcpus
> because Xeon phi chip already supports 288 logical cpus. As hardware
> development, there will be more logical cpus and we hope one guest can
> totally uses all cpu resources on the chip to meet HPC requirement.

Thanks, I think KVM will start by bumping the hard VCPU limit to 512 or
1024, with recommended maximum being 288.  You'll be able to raise the
hard limit just by configuing and recompiling.

>                             How about KVM X2APIC part? Do you have patch
> to extend KVM X2APIC to support 32-bit APIC ID?

I do, in limbo, as QEMU cannot create VCPUs with higher APIC IDs, yet.