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

[Zhang, Yang Z]

Paolo Bonzini wrote on 2014-12-19:
> 
> 
> 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

Agree, it is limited by hardware. But lowest priority distributes the interrupt more efficient than fixed mode. And current implementation more likes to switch the lowest priority mode to fixed mode. In case of interrupt intensive environment, this may be a bottleneck and VM may not benefit greatly from VT-d PI. But agree again, it is really a hardware limitation.

> 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.

The more easy way is to deliver the interrupt to the first matched VCPU we find. The round_robin_counter really helps nothing here since the interrupt is delivered by hardware directly.

> 
> Paolo
> 
>> 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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