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

[Wu, Feng]

> -----Original Message-----
> From: Paolo Bonzini [mailto:pbonzini@xxxxxxxxxx]
> Sent: Friday, December 19, 2014 12:58 AM
> To: Zhang, Yang Z; Wu, Feng; tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx;
> hpa@xxxxxxxxx; x86@xxxxxxxxxx; gleb@xxxxxxxxxx; dwmw2@xxxxxxxxxxxxx;
> joro@xxxxxxxxxx; alex.williamson@xxxxxxxxxx; 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 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.
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