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

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Date:	Thu, 18 Dec 2014 17:58:13 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Zhang, Yang Z" <yang.z.zhang@...el.com>,
	"Wu, Feng" <feng.wu@...el.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>
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



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

> 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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