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

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

On 2016-04-27 08:24, Lan Tianyu wrote:
> On 2016年04月27日 13:24, Jan Kiszka wrote:
>>>> If we don't want the interrupt from internal device delivers to CPU
>>>>>> 255, do we still need the VT-d interrupt remapping emulation? I think
>>>> firmware is able to send IPI to wakeup APs even without IR and OS is
>>>> able to do it too. So basically, only KVM and Qemu's support is enough.
> 
> Yes, just starting more than 255 APs doesn't need IR.
> 
>> What are "internal devices" for you? And which OS do you know that would
>> handle such artificial setups without prio massive patching?
>>
>> We do need VT-d IR emulation in order to present our guest a well
>> specified and support architecture for running > 255 CPUs.
> 
> Changing guest kernel will be big concern. I found commit ce69a784 did
> optimization to use X2APIC without IR in the guest when APIC id is less
> than 256 and so I proposed my idea to see everyone's feedback. Whether
> it's possible to relax the IR requirement when APIC id > 255 in the guest.

You can't do that easily because you can't address those additional CPUs
from *any* device then, only via IPIs. That means, Linux would have to
be changed to only set up IRQ affinity masks in the 0-254 range. I
suppose you would even have to patch tools like irqbalanced to not issue
mask changes via /proc that include larger CPU IDs. Practically not
feasible, already on Linux. Not to speak of other guest OSes.

Jan

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