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

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Message-ID: <5494131E.9030505@redhat.com>
Date:	Fri, 19 Dec 2014 12:59:26 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Wu, Feng" <feng.wu@...el.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Yang Zhang <yang.z.zhang@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>, Gleb Natapov <gleb@...nel.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"joro@...tes.org" <joro@...tes.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jiang Liu <jiang.liu-VuQAYsv1563Yd54FQh9/CA@...lic.gmane.org>
CC:	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Eric Auger <eric.auger@...aro.org>
Subject: Re: [v3 13/26] KVM: Define a new interface kvm_find_dest_vcpu() for
 VT-d PI



On 19/12/2014 02:30, Wu, Feng wrote:
>>> 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.

You can post lowest priority interrupts if they are delivered to a
single CPU, in which case they are effectively fixed priority.

If they are broadcast to multiple CPUs, do not post them.

Paolo
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