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

[Lan Tianyu <tianyu.lan@intel.com>]

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