linux-kernel - RE: [v3 13/26] KVM: Define a new interface kvm_find_dest

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Date:	Fri, 19 Dec 2014 01:47:54 +0000
From:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
To:	"Wu, Feng" <feng.wu@...el.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
	"gleb@...nel.org" <gleb@...nel.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"joro@...tes.org" <joro@...tes.org>,
	"alex.williamson@...hat.com" <alex.williamson@...hat.com>,
	"jiang.liu@...ux.intel.com" <jiang.liu@...ux.intel.com>,
	"Sankaran, Rajesh" <rajesh.sankaran@...el.com>
CC:	"eric.auger@...aro.org" <eric.auger@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [v3 13/26] KVM: Define a new interface kvm_find_dest_vcpu() for
 VT-d PI

Wu, Feng wrote on 2014-12-19:
> 
> 
> Paolo Bonzini wrote on 2014-12-19:
>> jiang.liu@...ux.intel.com
>> Cc: eric.auger@...aro.org; linux-kernel@...r.kernel.org;
>> iommu@...ts.linux-foundation.org; kvm@...r.kernel.org
>> 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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