RE: [v3 13/26] KVM: Define a new interface kvm_find_dest_vcpu() for VT-d PI

[Zhang, Yang Z]

Wu, Feng wrote on 2014-12-19:
> 
> 
> Paolo Bonzini wrote on 2014-12-19:
>> jiang.liu@xxxxxxxxxxxxxxx
>> Cc: eric.auger@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>> iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx
>> Subject: Re: [v3 13/26] KVM: Define a new interface
>> kvm_find_dest_vcpu() for VT-d PI
>> 
>> 
>> 
>> On 18/12/2014 15:49, Zhang, Yang Z wrote:
>>>>> Here, we introduce a similar way with 'apic_arb_prio' to handle
>>>>> guest lowest priority interrtups when VT-d PI is used. Here is
>>>>> the
>>>>> ideas: - Each vCPU has a counter 'round_robin_counter'. - When
>>>>> guests sets an interrupts to lowest priority, we choose the vCPU
>>>>> with smallest 'round_robin_counter' as the destination, then
>>>>> increase it.
>>> 
>>> How this can work well? All subsequent interrupts are delivered to
>>> one vCPU? It shouldn't be the best solution, need more consideration.
>> 
>> Well, it's a hardware limitation.  The alternative (which is easy to
>> implement) is to only do PI for single-CPU interrupts.  This should
>> work well for multiqueue NICs (and of course for UP guests :)), so
>> perhaps it's a good idea to only support that as a first attempt.
>> 
>> Paolo
> 
> Paolo, what do you mean by "single-CPU interrupts"? Do you mean we

It should be same idea as I mentioned on another thread: deliver the interrupt to a single CPU(maybe the first matched VCPU?)

> don't support lowest priority interrupts for PI? But Linux OS uses
> lowest priority for most of the case? If so, we can hardly get benefit
> from this feature for Linux guest OS.
> 
> Thanks,
> Feng
> 
>> 
>>> Also, I think you should take the apic_arb_prio into consider
>>> since the priority is for the whole vCPU not for one interrupt.


Best regards,
Yang


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