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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2016-04-29 04:09, Yang Zhang wrote:
> On 2016/4/28 23:32, Radim Krčmář wrote:
>> 2016-04-28 08:54+0200, Jan Kiszka:
>>> On 2016-04-28 03:11, Yang Zhang wrote:
>>>> On 2016/4/27 17:45, Jan Kiszka wrote:
>>>>> On 2016-04-27 11:39, Yang Zhang wrote:
>>>>>> I mean in Tianyu's case, if he doesn't care about to deliver external
>>>>>> interrupt to CPU >255, IR is not required.
>>>>>
>>>>> What matters is the guest OS. See my other reply on this why this
>>>>> doesn't work, even for Linux.
>>>>
>>>> Since there only few devices in his case, set the irq affinity manually
>>>> is enough.
>>
>> You could configure non-IPIs to work, but we want to create options that
>> are hard to break.
>>
>>> Ah, wait - are we talking about emulating the Xeon Phi architecture in
>>> QEMU, accelerated by KVM?
>>
>> Knights Landing will also be manufactured as a CPU, hopefully without
>> many peculiarities.
>>
>> I think we are talking about extending KVM's IR-less x2APIC, when
>> standard x2APIC is the future.
> 
> Yes, Since IR is only useful for the external device, and 255 CPUs is
> enough to handle the interrupts from external devices. Besides, i think
> virtual VT-d will bring extra performance impaction for devices, so if
> IR-less X2APIC also works well with more than 255 VCPUs, maybe extending
> KVM with IR-less x2apic is not a bad idea.

IR-less x2APIC for guest architectures that are expected to provide IR
remains a bad idea, at least until we have hard numbers what this
suspected performance impact actually is. Unless you update IRQ
affinities an insane rates, the impact should not be relevant because
remapping results are cached (for the irqfd hot-path) or you are already
taking the long way (userspace device emulation).

Jan

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